I would have said that the PRS-600 had the best 6" PDF viewer, until I read
Pocketbook 360 test diary. Now I am sure that PocketBook (same software on the 360 and the 301) is significantly better. It does not have a touch screen, and it does not have an arbitrarily located zoom - but this is of limited use on the PRS-600 because its setting are not remembered from page to page. The PRS-600 also does not have the option to autocrop margins (this has been added to the PRS-900)
I'm not convinced that any 6" EInk device is good enough for PDFs. The PocketBook 901 (9.7" screen) now looks very interesting, but I would actually like to see PocketBook's PDF capability in a device with a 8.1" screen.