Fonts: The most well-known feature in this context is probably the embedding of the fonts used. Every font used must also be embedded. If even the highest level a is to be achieved, each character must be marked with the corresponding Unicode name, even if no Unicode font is used at all. Colors: Colors should also not be left to chance or to the preference of the printer used. There are generally valid color models – this has nothing to do with PDF. These color standards should be used. And if this is not possible, then you should specify in the PDF file on which device it should preferably be output. Then a conversion to a generally valid color model can be done later. Encryption: PDF/A generally does not allow encryption. Also, in contrast to normal PDF, compression is not permitted throughout. Images can be compressed using a standardized process – such as Tiff, JPEG and, as of PDF/A-2, JPEG2000. More is not possible, because the contents of a PDF/A file should be accessible as easily as possible. The file must be protected against uncontrolled modification by a management system (DMS). In the case of document distribution, the use of digital rights management (DRM) is also an option. However, since this encrypts the files, a standardized PDF/A must still be kept on hand internally. Digital signatures: Digital signatures can be used to prove that a PDF file has not been modified after a certain point in time. PDF/A-1 did not make any statements about digital signatures. As a result, they were automatically allowed. And since PDF inherently allows handy embedded signatures, this feature was also popular. But only PDF/A-2 includes the reference to signature standards, so that errors in the use of signatures for long-term archiving are excluded here. Transparencies: In the original PDF reference, transparencies were described inaccurately. In the PDF ISO standard they are now so precise that they can also be used safely in long-term PDF. As of PDF/A-2, transparencies are therefore permitted. The ability to use transparencies is an important reason for switching to PDF/A-2 if you have previously created PDF/A-1. Scaling options: PDF/A-1 had a size limitation of 200 inches = 5.08m because of the PDF 1.4 base. This was too small for many applications. Scaling during PDF/A generation was the work-around. However, the original size was then no longer known in the PDF/A file itself. With PDF/A-2, a standard built-in scaling factor can now be used, the so-called user units. This means that extensions of around 300 km are now possible. Layers: In the past, the usability of layers in PDF/A was often missed. This PDF function is now also better standardized: a layer must be marked as a default layer. Then layers, which are helpful e.g. when converting from CAD models, can really be used. Embedding files: An important innovation of PDF/A-2 was the ability to embed other files. If these files are themselves PDF/A compliant again, this feature can be used. With PDF/A-3 it is even possible to embed arbitrary files. However, PDF/A-3 in no way guarantees any quality of the embedded files. For this you have to impose regulations on yourself, so that you don’t end up with a wildly uncompromising PDF/A-3. However, there are numerous applications for the possibility of embedding arbitrary files in PDF/A-3: 2D PDF views of a construction with stored original 3D geometry Invoice as PDF and stored invoice data as XML (ZUGFeRD) Originally existing non-PDF/A file but with valid digital signature embedded in the derived PDF/A file. Tagged files: Adobe has also created its own standard, the XMP standard, to mark files using XML tags. A file tagged in this way carries the information of what it is actually for within itself; a safeguard for the failure of the DMS system. XMP also includes several schemes for keywords. If the keywords you would like to use for your PDF/A archive are not included, simply create your own schema and embed it in the PDF/A file.