nupdf is a basic pdf reader, that i wrote for the dingoo a320/a330, and the qi-hardware nanonote. It uses the mupdf library to read the files, and displays most pdf files correctly.

There are some problems when using complex backgrounds with vector graphics overlaid ontop, and password protected pdfs aren't supported. Also as the library loads the entire file into memory at once, and as the target devices only have 32MB or so of ram, it can have problems with very large pdfs

qi-hardware wiki link
Dingoo file archive - nupdf v0.3