To not talk about the difficulty of use of a PDF viewer on smaller-screen Rockbox targets... like the Fuze v2!?
IMO viewing PDFs won't be too difficult on a bigger resolution device with touchscreen, like Cowon D2, but for devices with traditional keyboard and scrollwheel it would be a, at least, somewhat difficult task to read a single A4-size page... now imagine a bigger document (like the Rockbox Manual)...
(I know by experience what is viewing a PDF on embedded devices as I have contact with several touchscreen Windows Mobile mobile phones and PocketPCs that have a PDF viewer, and I can tell you that these devices have a bigger screen than the Cowon one)