Great program! it saved my day for reading comics, since the MOBI and EPUB readers don't seem to recognize "full screen" (also called panel view) epubs or mobis, as those produced by KindleComicConverter.
I personally use a program named Mangle to produce the cbz at the pocketbook's resolution, so that pages load quicker. Just select the "Kindle DX" profile, and there you go.
https://github.com/proDOOMman/Mangle
Looking at pbimageviewer's source code, I think it should not be very difficult to add PDF reading functionality, using the lightweight mupdf library, which basically renders a PDF page to a bitmap. The PDF contrast can be adjusted using a "gamma" parameter, and this would make a nice alternative PDF reader to the crappy reader that's included with the PocketBook (which has wrong gamma settings in my opinion).
The kindle community did a very nice integration of mupdf + CoolReader3 in a tool called kindlepdfviewer:
https://github.com/hwhw/kindlepdfviewer/wiki
Has anybody thought of integrating PDF reading to pbimageviewer, or do you think it should be a separate tool?