It looks like the directory structure has changed then in recent firmware versions, because when I open "Book" in my selector, it takes me to the "/mnt/ext1/" directory (I'm still using 2.0.4). I don't have either of the "Books" or "screens" directories. Oh well.
There is a long shot of producing PDF files on a device, but it would mean porting NetPBM and libtiff programs. If you can convert the images to TIFF format, then you can combine them into a single multi-page TIFF file using tiffcp, and then convert that to a PDF file using tiff2pdf. I suspect building those for the PocketBook devices wouldn't be hard. However, I don't know how to combine the PNG and SVG files into a single TIFF image (maybe using pngtopam, svgtopam, and then pamcomp to overlay them). With my non-touchscreen device and older firmware, I can't reproduce such notes myself to play around with.