[Nook Touch General] nmyshkin:
Most people would agree that the stock reader is not ideal for PDF books. Actually the PDF format is not really ideal for e-books, but that's another kettle of fish. The stock reader can make a lame attempt to reflow text-based PDFs, but when it is faced with image-based (generally scanned) PDFs it's really at sea.

(tip: want to hide PDFs from the stock Library? Put them in their own folder and include a ".nomedia" file. This prevents the stock library from seeing them but has no effect on at least the two other readers mentioned below)

So we turn to other readers. EBookDroid and PerfectViewer (and others) can display scanned PDFs with varying degrees of success. Of the two just mentioned, PerfectViewer probably does a better job. The files are often large and already taxing to the device but the engine in PerfectViewer seems well-equipped for this kind of thing--not surprising since it is really designed for image viewing. The one problem with PerfectViewer is that you can't smoothly scroll from page to page. Instead you move in chunks. Getting the right percentage chunk is tricky (you can adjust).

EBookDroid, on the other hand, can view any file as a continuous vertical scroll. The problem there is the engine used for PDFs. It's fine with text-based books, but it is seriously burdened by image-based PDFs. Pages take too long to load and crashes happen too often with large files.

My discovery is that the CBZ engine in EBookDroid is much faster and can read an image-based PDF readily if it is converted to CBZ (EBookDroid can also read CBR files--in theory--but the compression algorithm currently used for RAR files does not match what's in the old app, so stick with CBZ).

Converting PDF to CBZ

If you have the complete Adobe Acrobat software (as opposed to the free Reader) you can open the PDF and then export it as a series of jpg files. Zip these files and then change the file extension from .zip to .cbz

If you don't have the complete Acrobat there are many other options, but maybe one of the simplest is to do this online. Adobe itself offers this service: https://www.adobe.com/acrobat/online/pdf-to-jpg.html. You simply upload the PDF and then convert it. The image files are placed in a zip which you then download. Free. No need to open an account or sign in. Change the file name and extension and you're done.

Just to be up front, the files get larger. A 15 MB PDF became a 24 MB CBZ, as an example. But here, size matters not. It's how the reader app deals with the file type that matters. However, it

is

possible to significantly reduce the file size simply by reducing the individual page image size. Most scanned books I've come across have image sizes in the 1000s of pixels in both dimensions. That's not necessary for a good read. I have had good luck reducing the width to 600 pixels (the height adjusts proportionately). This is most convenient to do in a batch. Continue to the next section to see how.

Copy-protected PDFs pose an additional hurdle. I leave that to your own research.

Poor quality scans

You may have seen some. I've been looking at the much-maligned "Western Canon" of Harold Bloom lately and accumulating some of the works, many of which certainly qualify as "obscure". Some of these I have only been able to track down as scans of scholarly works, often from archive.org. They are frequently pretty reprehensible scans of everything from the outside and inside of the front cover to whatever else was there. The contrast is often poor and/or the background is far from white. "Fixing" such images is painful. I wanted something that could do the entire set of images in a batch and I found (free) Irfanview (https://www.irfanview.com/). Sorry, MacPeople, it's for Windows only but I'm sure there is something like it out there for you.

To decide what needs to be done I open a representative page image file and play with contrast, replacing the color of the dingy background, converting to grayscale, etc. Keep track of what you do, especially if you replace a color (copy down the RGB values). When the page looks "as good as it gets" you can do a batch conversion on the entire set of images (File>Batch conversion). By choosing the advanced options you will be presented with all of the many things you might have done to the image to make it better. Check those off and start the process. It's amazingly quick and you end up with perhaps not a perfect fix, but something that is much, much better. A before-and-after from "Prison of Love" by Diego de San Pedro attached below. Then you just zip the final images and change the file extension to .cbz