Digital
Preservation
Team

Preservation Assessment:
PDF Format (Part 2): PDF/A Profile

Date: 30/06/2019
Version: 1.0

PDF Format Preservation Assessment
Part 2: PDF/A Profile

Document History
Date
03/06/2019

Version
1.0

Author(s)Akiko Kimura, Peter May

Circulation
External(Restructured as a twopart document. See PDF
Format Preservation
Assessment, Part 1: PDF,
Version 1.5)

British Library Digital Preservation Team
digitalpreservation@bl.uk

This work is licensed under the Creative Commons Attribution 4.0 International License.

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DigitalPreservation
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Preservation Assessment:
PDF Format (Part 2): PDF/A Profile

Date: 30/06/2019
Version: 1.0

1. Introduction
This document provides a high level, non-collection specific assessment of the PDF/A file format
with regard to preservation risks and the practicalities of preserving data in this format.
This format assessment is one of a series of assessments carried out by the British Library’s
Digital Preservation Team. An explanation of criteria used in this assessment is provided initalics below each heading.
1.1 ScopeThis document is Part 2 of a two-part document. It will primarily focus on the most relevant (fordigital preservation) and commonly encountered sub-formats - or profiles - of the PDF family:
PDF/A (parts 1 through 3). This document should be read in conjunction with the core PDF(versions 1 through 2) assessment captured in the separate Part 1 document.
Note that this assessment considers format issues only and does not explore other factors
essential to a preservation planning exercise, such as collection specific characteristics, thatshould always be considered before implementing preservation actions.
1.2 PDF/A Summary
PDF/A is a profile of the PDF format. It is a multi-part ISO standard that restricts functionality
considered problematic for long-term archiving. Three parts of the standard have been publishedso far: PDF/A-1 through 3.
Restrictions placed across the PDF/A family include:







non-embedded fonts
JavaScript
audio and video content
LZW compression
non-embedded colour spaces, andencryption.

Each part of the PDF/A standard is an independent profile with various levels of conformance:
PDF/A-1 (ISO19005-1:2005) is based on PDF version 1.4 (not ISO standard). As wellas the above mentioned restrictions, PDF/A-1b prohibits additional functionality
including attachment of files in any formats, JPEG2000 compression and transparent
elements. Two levels of conformance are provided: level B (basic) – satisfies minimum
requirements necessary for visual appearance reproduction; level A (accessible) – adds
additional requirements on document structure in order to support text extraction andaccessibility (such as the use of Tagged PDF and Unicode character maps).
PDF/A-2 (ISO19005-2:2011) is based on PDF version 1.7 (ISO32000-1:2008). Unlike
PDF/A-1, it allows: attachment of other PDF/A files, JPEG2000 compression andtransparent elements. In addition to the conformance level B (basic) and level A(accessibility), PDF/A-2 provides level U – level B plus the requirement for all text tohave Unicode equivalents.
PDF/A-3 (ISO19005-3:2012) is also based on PDF version 1.7 and provides 3 levels ofconformance: A, B and U. The profile is basically identical to PDF/A-2 but with asignificant difference reflecting end-user/industry requirements — in particular, PDF/A3 allows attachment of files in any format.
In essence, PDF/A-1 is the most restrictive profile. The newer parts, PDF/A-2 and 3, haveadditional functions that came with the newer version of base PDF format as well as thereinstated functionally to attach other files (other PDF/As for A-2 and any format for A-3).

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2. Assessment
2.1 Development Status
A summary of the development history of the format and an indication of its current status
PDF/A was created against the backdrop of growing popularity of the PDF format, which is highly
complex and not suitable for long-term archiving of electronic documents. Wide-ranging
organisations such as administrative bodies, archiving communities and industries worldwide
pressed for the development of the standard, which resulted in the formation of an ISO Joint
Working Group and the release of ISO19005-1 (PDF/A-1) in 2005. The latest of the PDF/Afamily, PDF/A-3, was released in 2012. PDF/A-NEXT (aka PDF/A-4) is currently underdevelopment1.
2.2 Adoption and Usage
An impression of how widely used the file format is, with reference to use in other memoryorganisations and their practical experiences of working with the format
From the very start, the PDF/A standard has been recommended by many public and industrial
bodies as the archival format for electronic documents. Early adopters include the European
Commission, several European governments, and U.S. Courts [1]. The Library of Congress also
lists a growing number of organisations, many in the U.S., recommending or requiring PDF/A[2]. Despite the enthusiasm, the true picture of PDF/A adoption is difficult to quantify due partly
to the lack of reliable PDF/A identification/validation tools (discussed below). In her ‘DPC
Technology Watch Report’, Betsy Fanning suggests that, based on available figures, actual takeup of PDF/A format may be slow [3]2. This might be because end users are still preferring general
PDF format over PDF/A for its convenience and by necessity (e.g. academic reports requiring
inclusion of audio visual elements, attachment of data source files etc.). Also, even though
PDF/A-2 and 3 allow attachment of files, many tools currently claiming to create PDF/A(especially the ones aiming at non-enterprise users) only support creation of PDF/A-1 [2].
However, we may see a rapid change in PDF/A’s popularity depending on the improvement insoftware support and how the evolution of PDF/A-4 pans out.
2.3 Software Support
2.3.1 Rendering Software Support

An overall impression of software support for rendering the format with reference to: typicaldesktop software; and current support on British Library reading room PCs
Support for rendering is generally good as all PDF viewers are capable of rendering PDF/A files.
Not all PDF viewers are, however, conforming to the PDF/A standard. The ‘Whitepaper: A
technical introduction to PDF/A’ by PDFlib lists possible issues with non-conforming viewers, forexample, they may ignore embedded elements such as fonts and ICC-based colour spaces,
and use locally available options instead [4]3. In general, PDF viewers consider any attachedfiles as supplemental and do not attempt to render them. Conforming PDF/A viewers should,
however, provide a mechanism for attached files to be extracted and saved (but not necessarily
rendered) so that, if necessary, the files can be viewed on different software appropriate for theformat. Beyond that, how exactly a PDF/A conforming viewer should behave is still not welldefined. Such uncertainty can undermine the premise of PDF/A that it ‘provides a mechanism
for representing electronic documents in a manner that preserves their visual appearance overtime, independent of the tools and systems used for creating, storing or rendering the files 4’.

1 According to the placeholder on the ISO website, PDF/A-NEXT (ISO19005-4) is based on PDF version

2.0 (https://www.iso.org/standard/71832.html).
2 Page 16.
3 Page 4.
4 Quote from the introduction of the PDF/A-1 standard (ISO19005-1:2005).

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In addition, PDF/A viewers inherit any rendering issues native to PDF viewers owing in part tothe sheer complexity and range of functionality added to the PDF specifications over the years.
See the Part 1 assessment for further details on potential PDF rendering issues.
2.3.2 Preservation Software Support

An impression of the availability and effectiveness of software for managing and preserving
instances of the file format
Format identification
Being a profile of PDF, PDF/A is not readily distinguishable from the PDF specification it is basedon. Identification of the specific PDF/A variant could be considered more a matter ofcharacterisation and validation, and this is discussed below.
Validation, Conformance Checking and Detecting Preservation Risks
veraPDF [5], released in 2017, is the first complete open source PDF/A validator, covering allparts and conformance levels of the PDF/A standard. The development was initiated by the EUfunded PREFORMA project and led by the Open Preservation Foundation [6] and the PDF
Association [7]. Fanning describes veraPDF as ‘a new PDF/A validation tool that provides
comprehensive validity checks closely paired with precise format specification rules and a new
test suite’ and expresses her anticipation that ‘this development will help to remove ambiguities
and close loopholes in the format specification [3]5’. veraPDF also has a useful feature to validate
the conformance of documents against institutional policies.
In her 2018 thesis, Anna Oates conducted a case study in which she migrated a set of files fromvarious formats including PDF to PDF/A using various migration tools6. Of the 698 (supposedly)
successfully migrated files, only 483 files passed validation with veraPDF [8]7. It should be notedthat the scope of veraPDF is restricted to the clauses within the PDF/A standard themselves
due to the limited resources. It means that some elements only described in the PDF
specifications are not validated to the full extent but only in the manner in which conformance tothe PDF/A standard is satisfied. Examples of affected functionality include JPEG 2000, ICC
profiles, and Tagged PDF. Users are encouraged to use veraPDF’s extensible architecture todevelop necessary plugins to fulfil their own needs for PDF/A validation [9]8. It should be notedthat some issues affecting rendering of the content, such as corrupt images, are out of the scopeof validation tools including veraPDF.
Up until the release of veraPDF, the lack of effective PDF/A validation tools has been highlighted
in several experiments: Florida Virtual Campus reported on the shortcomings of JHOVE PDF/A
validation [10]. PDF/A Manager [11] (part of the PDF Tron suite [12]9) was assessed (along with
pdfaPilot [13] and 3-Heights [14]) by Carol Chou and Jamin Koo, with all products achieving
PDF/A validation accuracy results of between 90 and 95% [15]. Also, Jo Gilham and Peter Cliff
reported disparity between the validation results of different tools such as PDF/A Manager and
Adobe Preflight [16] [17]. More independent case studies to better ascertain the comparative
accuracy and effectiveness of veraPDF would be extremely beneficial.
Metadata Extraction
Any metadata extraction tools which handle PDFs, such as Apache Tika [18], the NLNZ
Metadata Extraction Tool [19], JHOVE [20], or 3-Heights™ PDF Extract [21] should beapplicable to PDF/A files.

5 Page 14.
6 The migration tools Oates used in her case study were: Adobe Acrobat DC, Callas pdfaPilot, Intarsys

PDF/A Live!, LibreOffice, PDF Studio, PDFForge PDFCreator, and PDFTron PDF/A Manager CMD
migration software.
7 Page 32.
8 Page 161.
9 PDF/A Manager is a commercial offering providing PDF validation used by the Kost-val [70] toolset at
Kost [53].

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Content Extraction
Reusability of the content of a PDF/A document is a concern for memory organisations. As PDF
was developed fundamentally as the format for visual representation of page-based documents,
extraction of text and other PDF objects to reuse them on different format/media is a challenge.
Various types of assistive technology (AT) introduced to assist visually impaired users, such as
Tagged PDF, could also support content of the document to be extracted in a semantically
logical order, and helps search engines to discover the content reliably. The PDF/A standard’s
conformance level A (accessible) clearly indicates that such AT measures were applied to the
document, which could pave the way for effective content extraction tools for PDF/A content tobe developed. However, Duff Johnson argues that the level A conformance of PDF/A is totally
insufficient: ‘Technically, it's possible to comply with PDF/A-1a with a single tag for each page,irrespective of the document's contents. That's the key reason why claims of conformance orvalidation of PDF/A-1a are, by themselves, essentially meaningless’ [22]10.
Migration
Migration from PDF/A to other formats may be greatly assisted by the fact that the source files
are not encrypted and some elements, such as fonts and colour spaces, are embedded in thefiles to prevent the loss of key components of the documents.
There are a number of dedicated PDF focused applications which support migration to and from
PDF/A. The PDF Association maintains a list of its members’ relevant products [23]. Not all thetools, however, support all the variants of PDF/A. Even some popular products of Adobe, theinventor of the PDF format, only support export to PDF/A-1b (e.g. InDesign version 14.0 [24]).
Support for quality checking via comparison between source and destination files is generally
poor, and may not provide sufficient confidence that significant properties have survived thetransformation. Fanning adds particular issues concerning migration to PDF/A which include: aloss of embedded digital signatures, attached files, transparency, and also a scenario that, whenfonts in the source files are not available to embed, a migration tool may make a poor fontsubstitution which may result in not only a loss of appearance of the document but also loss of
meaning if specialised fonts such a mathematical font was substituted without sufficient warning[3]11. Jenny Mitcham also notes various challenges with PDF to PDF/A migration [25].
As discussed above, the nature of the restrictions in PDF/A means that wholesale migration ofa PDF collection to PDF/A is unwise. Before committing to any file migration, thought should begiven as to the aspects of the original file that must be preserved, and whether or not PDF/A cansupport them.
2.4 Documentation and Guidance
An indication of the availability of practical documentation or guidance with specific reference tothe facilitation of any recommended actions
All the parts of PDF/A (1 through to 3) are ISO standards and the documentation is available fordownload (with a fee for non-ISO members). PDF 1.7, which is the underlying format of PDF/A2 and 3, is also an ISO standard, and the copies of the specifications are provided by Adobe[26] along with an archive of legacy documentation [27]. PDF/A-1 is based on PDF 1.4, which is
not an ISO standard but its documentation ‘PDF Reference, third edition, Adobe Portable
Document Format, Version 1.4 [28]’ is made freely available to the public by Adobe. In addition,
support for PDF and PDF/A in general is available from a number of sources, such as the PDF
Association.
As mentioned in Part 1, PDF viewers’ tolerances to invalid PDF files is often considered as aconsequence of ambiguities in PDF documentation over time. While this is a matter affecting all
the current PDF/A variants, PDF/A-4 (currently under development) should benefit from the fact
10 Johnson goes on to discuss PDF/UA (Universal Accessibility) which sets a clear standard to describe

accessible PDF in technically complete terms: ‘a really good PDF/A-1a file is one that also complies with
PDF/UA’.
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that it is based on PDF 2.0 (ISO 32000-2) released in 2017. According to Johnson, PDF 2.0 ‘has
evolved’ in terms of clarity in its documentation [29].
2.5 Complexity
An impression of the complexity of the format with respect to the impact this is likely to have on
the British Library managing or working with content in this format. What level of expertise in theformat is required to have confidence in management and preservation?
PDF/A is designed to overcome the long-term preservation risks identified in the PDF format.
Nonetheless, PDF/A is fundamentally a PDF and therefore inherits the issues derived from
PDF’s complexity and uncertainty (see more in the Part 1 assessment). In particular, PDF/A-3
files require careful handling because of its file attachment functionality as discussed below.
2.6 Embedded or Attached Content
The potential for embedding or attaching files of similar or different formats, and the likely
implications of thisPDF/A-3 removes a single but contentious restriction to the PDF/A-2 standard by allowing any
file format to be embedded as a file stream. This may prove beneficial in some use cases, andespecially from a preservation point of view where, for example, a source document for the
PDF/A itself can be attached. It has been observed, however, the attachment facility has thepotential to be used for a variety of purposes, such as embedding additional information or eveninformation of a more critical nature than the primary document. This leaves potentially difficult
questions for an archive. Is an attached file of critical importance for preservation (despite
potentially being of any format and hence potentially a significant preservation risk), merely asecondary object with optional or additional data, or, as noted above, the source data for the
primary document?
Given such concerns, PDF/A-3 standard imposes additional requirements for attached files(termed ‘associated files’ in the standard) including: each embedded file stream has to have aheader stating the MIME type (although ‘application/octet-bitstream’ can be used if a precise
MIME type is not known); relationships between attached files and the primary PDF/A-3 file must
be explicitly expressed within the file specification dictionary (predefined key values are: source,
data, alternative, supplement or unspecified) [30]. Although such measures could help
preservation tools and procedures to improve, would PDF/A creators take the time to providethat accurate metadata where ‘unknown/unspecified’ statements are allowed? How will the
PDF/A creating software influence this process?
veraPDF can be configured to detect PDF/As with file attachments. Although it is a significant
improvement from the PDF/A collection management point of view, safeguarding such files
tacked inside the PDF/A container remains a challenge for archiving organisations. The NDSAPDF/A-3 Working Group published a detailed discussion focused on these concerns [31], which
considers a number of use cases and makes some sensible recommendations (such as
checking received PDF/A-3 files for embedded files, and treating PDF/A-3 files separately fromthe other PDF/A variants in terms of format preference lists and related format action plans). On
the whole, however, this remains a significant area of concern.
Also it needs to be mentioned that, the size of a PDF/A-2 or 3 file can be significantly large due
to file attachments. Even with no attachment, a PDF/A file may be larger than a PDF file of thesame content due to embedded fonts etc. According to the PDF Association, ‘Embedded fonts
can slightly increase the size of a PDF/A file’, and it can be in some cases problematic whenarchiving a very large number of documents [32]12. On the other hand, Fanning notes that
‘PDF/A-2 introduced compressed objects and XRef streams to minimize file sizes [3]13’. The true
impact of file size issues for archiving organisations remains to be seen.

12 Page 18.
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2.7 External Dependencies
An indication of the possibility of content external to an instance of the file format that iscomplimentary or even essential to the intellectual content of the instance
Removing external dependencies is one of the key measures applied to the PDF/A standard inan effort to reduce preservation risks. Key components such as fonts, colour space, and images
are required to be embedded in PDF/A. However, there is a common misconception that PDF/A
does not allow any external references at all; Johan van der Knijff notes, ‘contrary to some(incorrect) popular belief [sic], the PDF/A standards do not rule out references to external files
completely. The following mechanisms for referring to external content are allowed: URI actions:these refer to Internet resources (i.e. a clickable hyperlink), GoToR actions: these refer to an
external PDF file (i.e. a clickable link to a locally stored PDF) … From a preservation point ofview, GoToR actions may nevertheless be a risk (e.g. in case of a collection of PDFs that referto each other), even though the rendering of the files that contain the reference is not affected’[33].
2.8 Legal Issues
Legal impediments to the use, management or preservation of instances of the file format
As far as the PDF/A format is concerned, there is no conspicuous legal issue relating topreservation risks. All variants of PDF/A profiles are ISO standards as well as PDF 1.7, which
PDF/A-2 and 3 are based on. Even though patents relating to PDF 1.4, which PDF/A-1 is basedon, is owned by Adobe, the company has issued royalty free rights on a significant number ofpatents in order to encourage uptake and third party development of PDF tools [34]. And achange of policy in at least the short term appears to be highly unlikely.
A potential legal issue with PDF/A creation/migration is copyright restriction with fonts. The
PDF/A standard requires fonts to be embedded in the document to reduce preservation risks.
However, the requirement may unwittingly open up other types of preservation risk. Firstly,
copyrighted specialised fonts may prevent electronic documents to be saved as PDF/A at all.
Chou and Koo note that: ‘There are ways to circumvent possible copyrights infringement throughfont substitution but some specialized fonts may prove to be difficult not only to procure but also
to use in PDF/A conversion, as their makers can prohibit embedding of fonts’ [15]. Secondly,
where a restricted font has been embedded without copyright clearance, this may risk copyrightinfraction for the preserving organisation. Fanning suggests that there may also be restrictions
on how fonts are distributed: ‘Organizations sometimes use their own bespoke families of fonts
to help authenticate documents. These restrictions may impact on where PDF/A can be applied,
how it can be preserved and where access can be provided’ [3]14. Furthermore, as mentioned
earlier, a PDF/A migration tool may make a poor font substitution and embed copyright-protected
fonts into the document without issuing a sufficient warning.
2.9 Technical Protection Mechanisms
Encryption, Digital Rights Management and any other technical mechanisms that might restrict
usage, management or preservation of instances of the file format
There is no encryption issue with PDF/A as the functionality is prohibited in the standard.
2.10 Other Preservation Risks
Other evidence based preservation risks, noting that many known preservation risks are format
specific and do not easily fit under any of the sustainability factors above
All the variants of PDF/A standard permits embedded digital signatures, while the use of the
PDF Advanced Electronic Signatures (PAdES) standard is permitted in PDF/A-2 and 3. Digital
signatures are a mixture of PDF objects and strings in a cryptographic message syntax [35].
According to Fanning, migration to PDF/A will ‘break’ the signature [3]15.

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Unlike migration, receipt or deposit of PDF/A-1 and 2 files may not raise significant preservation
concerns as the standard prohibits functionality associated with the preservation risks identified
in this report. Assuming, of course, that the source is trusted and the files do indeed conform to
the restrictions described in the standard.
2.11 Preservation Risk Summary
A summary of preservation risks and recommended actions (where possible)
PDF/A is a widely recognised profile of PDF, the world’s most ubiquitous electronic document
format, designed for long-term archiving by restricting specific functionality identified aspreservation risk.
Although some of the most prominent risks for preservation identified in the PDF assessment(Part 1 of this document) such as encryption and missing fonts are eliminated from PDF/A, it still
inherits deep-seated complexity and uncertainty from the underlying PDF format. Many
preservation issues may be unnoticed as a result of viewers tolerating invalid or badly formed
files. Unlike earlier flavours of PDF/A, PDF/A-3’s capability to have file attachments in any
formats reintroduced the risk for potentially key elements of the document being unidentified
and/or lost during the migration process. Furthermore, there is no guarantee that software to
render such attachments in various formats is available now, or in the future.
The definition as to what a PDF/A conforming software should consist of is poorly defined
meaning, for example, a mathematical font embedded in PDF/A may be substituted with a locallyavailable font by the viewing tool, potentially impairing the visual appearance and understandingof the document. Similarly, when migrating to a PDF/A, the tool may not be clear when acopyright protected font has been embedded in the document, which risks copyright infraction
for the preserving organisation.
Although the lack of reliable PDF/A validation tools have been a major issue widely recognised
within digital archiving communities, the recent development of veraPDF has changed thelandscape. It has made it possible for an archiving organisation to assess its PDF/A collections
more reliably, and plan for effective long-term preservation strategies.
The following list summarises risks for PDF/A.


Invalid or badly formed PDF files
o May affect ability to render files now or in the future

Legal issues
o Embedded copyrighted fonts may pose copyright infraction threats for thepreserving organisationFile attachmentso Attachments of any file format could pose a variety of preservation risks inthemselves (PDF/A-3 only)File sizeo Embedded fonts and colour spaces, and attached files (PDF/A-2 and 3 only) may
increase the file size

Loss of significant properties
o Embedded video, sound, JPEG2000 streams16, digital signatures, and/or interactive
elements such as JavaScript will be discarded on conversion to PDF/Ao Attached files may be discarded on conversion to PDF/A-1 and 2External referenceso References to locally stored PDF files (via GoToR action) may affect rendering if
they change or move the location.

16 Note that JPEG2000 compression is forbidden in PDF/A-1 but not in PDF/A-2 and 3.

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3. Recommendations for Action
Recommended actions in usage and handling of the format. Recommend actions in the support
or development of software applications that provide, or have the potential to provide, significant
risk mitigation for the format. Note that these recommendations do not take into account otherrequirements such as those driven by specific British Library collections, or non-preservation
issues such as resourcing.
As software support for PDF/A has improved, most notably with veraPDF, there are more
opportunities to mitigate preservation risks identified in this assessment. This section should beread in conjunction with the Recommended Action section in the core PDF assessment (Part 1).
Handling Recommendations









It is recommended that PDFs are created to one of the PDF/A standards (ideally PDF/A1) and validated using a suitable PDF validator.
Batch migration from PDF to PDF/A, without deep analysis of the collection, is notrecommended. It could potentially do more harm than good, and risk the loss ofsignificant properties.
Receipt or deposit of PDF/A is recommended to prefer the PDF/A-1 profile rather than
PDF/A-2 and 3 to reduce the risk concerning attached files.
It is recommended that PDF/As are validated with a suitable PDF validator at the pointof receipt or deposit to check their conformity to the PDF/A standards.
Files confirmed to be PDF/A-3 with file attachments should be, where possible,
managed in such a way that specific preservation actions can be applied. Files should
be checked with a suitable PDF validator to confirm the presence of embedded files, but
be aware the PDF user-created metadata (see below).
User-created metadata concerning attached files (MIME type and relationships to theprimary document) should not be trusted; deeper inspection should be performed.
Digitally signed documents should not be migrated to PDF/A, or should be re-signed
after migration [3]17.

Knowledge Recommendations

Understand the frequency and impact of PDF/A risks in specific collections in order to
identify genuine problems, or discount identified risks whose frequency or impact is not
significant.
Undertake independent case studies to better ascertain the comparative accuracy andeffectiveness of validation tools, especially veraPDF.

Software RecommendationsSupport enhancements to validation tools to improve detection of attached files (e.g.
MIME type).
Support enhancements to migration tools to improve detection of significant properties
lost or substituted during the process of transformation.

Monitoring Recommendations
The preservation risks faced by PDF/A are unlikely to change rapidly and so review of thisdocument should not be considered a high priority. However awareness of new softwaredevelopments, particularly to veraPDF, and to new PDF/A variants should be maintained: Monitor veraPDF for new features and capabilities.
 Monitor development of PDF/A-4.

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4. References
1. PDF/A archiving standard. Adobe. [Online] 23 12 2012. [Cited: 19 June 2019.]
http://web.archive.org/web/20121223184242/http://www.adobe.com/enterprise/standards/pdfa/.
2. PDF/A Family, PDF for Long-term Preservation. Library of Congress. [Online] [Cited: 19
June 2019.] https://www.loc.gov/preservation/digital/formats/fdd/fdd000318.shtml.
3. Fanning, Betsy A and AIIM. Preservation with PDF/A (2nd Edition). s.l. : Digital Preservation
Coalition, 2017. ISSN: 2048-7916.
4. PDFlib GmbH. Whitepaper: A Technical Introduction to PDF/A. s.l. : PDFlib, 2013. p. 4.
5. veraPDF. veraPDF. [Online] [Cited: 18 June 2019.] https://verapdf.org/home/.
6. Open Preservation Foundation. [Online] [Cited: 19 June 2019.]
https://openpreservation.org/.
7. PDF Association. [Online] [Cited: 24 February 2015.] http://www.pdfa.org/.
8. Oates, Anna. Navigating the PDF/A Standard: A Case Study of Theses in the University of
Oxford's Institutional Repository. s.l. : Graduate College of the University of Illinois at UrbanaChampaign, 2018.
9. veraPDF: building an open source, industry supported PDF/A validator for cultural heritage
institutions. Wilson, Carl, McGuinness, Rebecca and Jung, Joachim. 2, s.l. : Emerald, 2017,
Digital Library Perspectives, Vol. 33.
10. PDF-A Validation and Conversion in Florida Digital Archive. Florida Virtual Campus.
[Online] 19 September 2013. [Cited: 20 June 2019.]
https://libraries.flvc.org/documents/181844/502298/PDFA+Validation+and+Conversion+in+FDA/.
11. PDF/A Manager. PDFTron. [Online] [Cited: 20 June 2019.]
https://www.pdftron.com/documentation/cli/guides/pdfa-manager/overview/.
12. PDFTron: Functionality. PDFTron. [Online] [Cited: 20 June 2019.]
https://www.pdftron.com/pdf-sdk/features.
13. pdfaPilot. Callas Software. [Online] [Cited: 20 June 2019.]
https://www.callassoftware.com/en/products/pdfapilot.
14. 3-Heights PDF Validator – PDF and PDF/A standard conformance validation. PDF Tools.
[Online] [Cited: 18 June 2019.] http://www.pdf-tools.com/pdf20/en/products/pdf-convertervalidation/pdf-validator/.
15. PDF to PDF/A: Evaluation of Converter Software for Implementation in Digital Repository
Workflow. Koo, Jamin and Chou, Carol C. H. 2013. iPRES 2012. pp. 4-5.
https://ipres.ischool.utoronto.ca/sites/ipres.ischool.utoronto.ca/files/iPres 2012 Conference
Proceedings Final.pdf.
16. Adobe. Correcting problem areas with the Preflight tool (Acrobat Pro). Adobe. [Online][Cited: 18 June 2019.] https://helpx.adobe.com/acrobat/using/correcting-problem-areaspreflight-tool.html.
17. Gilham, Jo and Cliff, Peter. PDFA SPRUCE Scenario. OPF Wiki. [Online] 2013. [Cited: 24
February 2015.] http://wiki.opflabs.org/display/SPR/PDFA+Validation+tools+give+different+results.
18. Apache Tika. Apache Software Foundation. [Online] [Cited: 20 June 2019.]
http://tika.apache.org.
19. Metadata Extraction Tool. National Library of New Zealand. [Online] [Cited: 20 June 2019.]
http://meta-extractor.sourceforge.net/.
20. JHove. JHove Sourceforge. [Online] [Cited: 20 June 2019.]
http://sourceforge.net/projects/jhove/.

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21. 3 Heights(TM) PDF Extract. PDF-Tools. [Online] [Cited: 19 June 2019.] http://www.pdftools.com/pdf/pdf-extract-content-metadata-text.aspx.
22. Johnson, Duff. Accessibility: What PDF/A-1a Really Means. PDF Association. [Online] 1
October 2010. [Cited: 26 June 2019.] https://www.pdfa.org/accessibility-what-pdf-a-1a-reallymeans/.
23. About our members products. PDF Association. [Online] [Cited: 20 June 2019.]
https://www.pdfa.org/products/.
24. Adobe InDesign User Guide: Export to Adobe PDF. Adpbe. [Online] [Cited: 20 June 2019.]
https://helpx.adobe.com/uk/indesign/using/exporting-publishing-pdf.html.
25. Mitcham, Jenny and Wheatley, Paul. PDF to PDF-A conversion. OPF Wiki. [Online] 2012.
[Cited: 20 June 2019.] http://wiki.opf-labs.org/display/REQ/PDF+to+PDF-A+conversion.
26. PDF Reference and Adobe Extensions to the PDF Specification. Adobe. [Online] [Cited: 24
February 2015.] http://www.adobe.com/devnet/pdf/pdf_reference.html.
27. Adobe PDF Reference Archives. Adobe. [Online] [Cited: 24 February 2015.]
http://www.adobe.com/devnet/pdf/pdf_reference_archive.html.
28. Adobe Systems Incorporated. PDF Reference, third edition: Adobe Portable Document
Format, Version 1.4. s.l. : Adobe Systems Incorporated, 2001. ISBN: 0-201-75839-3.
29. Johnson, Duff. PDF 2.0: The worldwide standard for electronic documents has evolved.
PDF Association. [Online] 30 August 2017. [Cited: 12 June 2019.] https://www.pdfa.org/pdf-20-the-worldwide-standard-for-electronic-documents-has-evolved/.
30. Library of Congress. PDF/A-3, PDF for Long-term Preservation, Use of ISO 32000-1, With
Embedded Files. [Online] [Cited: 25 June 2019.]
https://www.loc.gov/preservation/digital/formats/fdd/fdd000360.shtml#notes.
31. NDSA Standards and Practices Working Group. The Benefits and Risks of the PDF/A-3
File Format for Archival Institutions. Digital Preservation. [Online] February 2014. [Cited: 25
June 2019.]
http://www.digitalpreservation.gov/ndsa/working_groups/documents/NDSA_PDF_A3_report_fi
nal022014.pdf.
32. Oettler, Alexandra. PDF/A in a Nutshell 2.0: PDF for long-term archiving. Berlin :Association for Digital Document Standards, 2013. pp. 13-15.
33. PDF Format Issues: References to external files. OPF Wiki. [Online] [Cited: 24 June25
2019.] http://wiki.opf-labs.org/display/TR/References+to+external+files.
34. Library of Congress. PDF (Portable Document Format) Family. Sustainability of Digital
Formats. [Online] [Cited: 25 June 2019.]
http://www.digitalpreservation.gov/formats/fdd/fdd000030.shtml.
35. Bärfuss, Hans. Digital signatures in PDF/A. PDF-TOOLS. [Online] 25 June 2019.
http://blog.pdf-tools.com/2015/01/digital-signatures-in-pdfa.html.
36. ISO/DIS 32000-2 PDF 2.0. ISO. [Online] [Cited: 24 February 2015.]
http://www.iso.org/iso/catalogue_detail.htm?csnumber=63534.
37. Albertini, Ange. A PDF 101 document walk through. [Online] [Cited: 24 February 2015.]
http://imgur.com/a/PbN8H#7.
38. Portable Document Format. Wikipedia. [Online] [Cited: 24 February 2015.]
http://en.wikipedia.org/wiki/Portable_Document_Format.
39. Johnson, Duff. The 8 most popular document formats on the Web. Duff Johnson Strategy
and Communications blog. [Online] 17 February 2014. [Cited: 24 February 2015.] http://duffjohnson.com/2014/02/17/the-8-most-popular-document-formats-on-the-web/.
40. Characterising and Preserving Digital Repositories: File Format Profiles. Hitchcock, Steveand Tarrant, David. 66, January 2011, Ariadne. http://eprints.soton.ac.uk/273241/.

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41. Formats over Time: Exploring UK Web History. Jackson, Andrew N. Toronto : s.n., 2012.
iPRES 2012. http://arxiv.org/abs/1210.1714.
42. Johnson, Duff. PDF Readers - 5 readers compared. Talking PDF. [Online] 30 November
2010. [Cited: 24 February 2015.] http://talkingpdf.org/pdf-readers-5-readers-compared/.
43. List of PDF Software. Wikipedia. [Online] [Cited: 24 February 2015.]
http://en.wikipedia.org/wiki/List_of_PDF_software.
44. Adobe Reader XI. Adobe. [Online] [Cited: 24 February 2015.]
http://www.adobe.com/uk/products/reader.html.
45. Migration: Context and Current Status. Digital Preservation Testbed. [Online] 5 December
2001. [Cited: 24 February 2015.]
http://www.nationaalarchief.nl/sites/default/files/docs/kennisbank/migration_0.pdf.
46. The Network is the Format: PDF and the Long-term Use of Digital Content. Morrissey,
Sheila. Copenhagen, DK : s.n., 2012. Archiving 2012. Vol. 8, pp. 200-203.
http://www.portico.org/digital-preservation/wpcontent/uploads/2012/12/Archiving2012TheNetworkIsTheFormat.pdf.
47. Johnson, Duff. Are Your Documents Readable? How Would You Know? Duff Johnson
Strategy and Communications Blog. [Online] 24 January 2014. [Cited: 24 February 2015.]
http://duff-johnson.com/2014/01/24/are-your-documents-readable-how-would-you-know/.
48. van der Knijff, Johan. Identification of PDF Preservation Risks: Analysis of GovDocs
Selected Corpus. OPF Blog. [Online] 27 January 2014. [Cited: 24 February 2015.]
http://www.openplanetsfoundation.org/blogs/2014-01-27-identification-pdf-preservation-risksanalysis-govdocs-selected-corpus.
49. JHove PDF-hul Module. JHove Sourceforge. [Online] [Cited: 24 February 2015.]
http://jhove.sourceforge.net/pdf-hul.html.
50. Owens, Evan. Automated Workflow for the Ingest and Preservation of Electronic Journals.
[Online] 2010?? http://www.portico.org/digital-preservation/wpcontent/uploads/2010/01/Archiving2006-Owens-pres.pdf.
51. SPRUCE Characterisation Hackathon. OPF Wiki. [Online] 2013. [Cited: 24 February 2015.]
http://wiki.opflabs.org/display/SPR/SPRUCE+Hackathon+Leeds%2C+Unified+Characterisation.
52. Cliff, Peter. Visual Analysis of PreFlight Output. OPF Wiki. [Online] [Cited: 24 February
2015.] http://wiki.opf-labs.org/display/SPR/Visual+Analysis+of+Preflight+Output.
53. KOST-CECO. [Online] [Cited: 24 February 2015.] http://kost-ceco.ch/cms/.
54. 3-Heights(TM) PDF to PDF/A Converter. PDF Tools. [Online] [Cited: 24 February 2015.]
http://www.pdf-tools.com/pdf/pdf-to-pdfa-converter-signature.aspx.
55. Johnson, Duff. PDF Validation Dream or Yawn? Waking up to the possibilities of an opensource PDF validator. Duff Johnson Strategy and Communications Blog. [Online] 2013. [Cited:
24 February 2015.] http://duff-johnson.com/wpcontent/uploads/2014/01/PDFValidationDreamOrYawn.pdf.
56. Flint. Github. [Online] [Cited: 24 February 2015.] http://openpreserve.github.io/flint/.
57. Shea, Dan. Acrobat and PDF Developer Libraries. Planet PDF. [Online] 13 November
2013. [Cited: 24 February 2015.]
http://www.planetpdf.com/developer/article.asp?ContentID=acrobat_pdf_developer_librar&gid=6218.
58. QPDF. QPDF Sorceforge. [Online] [Cited: 24 February 2015.] http://qpdf.sourceforge.net/.
59. Johnson, Duff. Is PDF an Open Standard? Planet PDF. [Online] 10 June 2010. [Cited: 24
February 2015.]
http://www.planetpdf.com/enterprise/article.asp?ContentID=Is_PDF_an_open_standard&page=1.

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60. Wolf, Julia. OMG-WTF-PDF Denouement. FireEye. [Online] 2 February 2011. [Cited: 24
February 2015.] http://www.fireeye.com/blog/technical/cyber-exploits/2011/02/omg-wtf-pdfdenouement.html.
61. van der Knijff, Johan. What do we mean by "embedded" files in PDF? OPF Blog. [Online] 9
January 2013. [Cited: 24 February 2015.] http://www.openplanetsfoundation.org/blogs/201301-09-what-do-we-mean-embedded-files-pdf.
62. Wolf, Julia. OMG-WTF-PDF [PDF Ambiguity and Obfuscation]. Troopers. [Online] 31
March 2011. [Cited: 24 February 2015.] https://www.troopers.de/wpcontent/uploads/2011/04/TR11_Wolf_OMG_PDF.pdf.
63. Albertini, Ange. PDF Tricks. Corkami. [Online] 2014. [Cited: 24 February 2015.]
https://code.google.com/p/corkami/wiki/PDFTricks.
64. Detect, extract and analyse embedded objects in PDFs. OPF Wiki. [Online] 2011. [Cited:
24 February 2015.] http://wiki.opflabs.org/display/AQuA/Detect%2C+extract+and+analyse+embedded+objects+in+PDFs .
65. PDF Format Issues: JavaScript . OPF Wiki. [Online] [Cited: 24 February 2015.]
http://wiki.opf-labs.org/display/TR/JavaScript.
66. PDF Format Issues: Fonts missing, damaged or incomplete. OPF Wiki. [Online] [Cited: 24
February 2015.] http://wiki.opflabs.org/display/TR/Fonts+missing%2C+damaged+or+incomplete.
67. SPRUCE PDF Solution: PDF Characterisation Tool. OPF Wiki. [Online] 2011. [Cited: 24
February 2015.] http://wiki.opf-labs.org/display/AQuA/PDF+Characterisation+Tool.
68. Born Broken: Fonts and Information Loss in Legacy Digital Documents. Brown, Geoffrey
and Woods, Kam. 1, s.l. : University of Edinburgh, 2011, International Journal of Digital
Curation, Vol. 6, pp. 5-19. http://dx.doi.org/10.2218/ijdc.v6i1.168. ISSN 1746-8256.
69. PDF Current Threats. Malware Tracker. [Online] [Cited: 25 February 2015.]
http://www.malwaretracker.com/pdfthreat.php.
70. KOST-Val. Community Owned digital Preservation Tool Registry (COPTR). [Online] [Cited:
20 June 2019.] http://coptr.digipres.org/KOST-Val.
71. Archaeology Data Service. [Online] [Cited: 20 June 2019.]
http://archaeologydataservice.ac.uk/.
72. Archaeology Data Service. [Online] [Cited: 24 February 2015.]
http://archaeologydataservice.ac.uk/.
73. Adobe PDF Reference Archives. Adobe. [Online] [Cited: 24 June 2019.]
http://www.adobe.com/devnet/pdf/pdf_reference_archive.html.
74. PDF Reference and Adobe Extensions to the PDF Specification. Adobe. [Online] [Cited: 24
June 2019.] http://www.adobe.com/devnet/pdf/pdf_reference.html.
75. Johnson, Duff. Archivists: No flowers for PDF/A-3. Duff Johnson's Strategy andCommunications Blog. [Online] 28 February 2014. [Cited: 25 June 2019.] http://duffjohnson.com/2014/02/28/archivists-no-flowers-for-pdfa-3/.
76. van der Knijff, Johan. Adobe Portable Document Format: Inventory of long termpreservation risks. Open Preservation Foundation. [Online] 20 October 2009. [Cited: 25 June
2019.]
http://www.openplanetsfoundation.org/system/files/PDFInventoryPreservationRisks_0_2_0.pdf.
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