To add to Rizla's suggestion,
this long (37+ minutes) youtube video comparing the 6.8" H2O and the 8" Pocketbook Inkpad 840 for PDFs prefers the H2O with KOReader.
If you go 6", and don't mind preparing each PDF,
k2pdfopt is quite nice. That's what I use to optimize PDFs for viewing (mostly in landscape) on my locked-down PW2.