The paperwhite says that it supports pdf natively. The question is if that includes image pages and how easy or difficult is to get them onto the kindle. It seems that the paperwhite does not have email capability.
All four (kindle, Paperwhite, 2 fires) can download documents and have internet capabilities for searching and browsing only (fires can download also)
Only fires have color, email, video, speakers, and downloading mp3 audio shiurim capabilities.
CMIIW, but I think only the HDX has a mic
kindle vs. PaperwhiteNOTE: These offers are for the small ~6.5" tablets and 7" fire tablets, not the larger ones.