If your pdf's are not in A4 format (21 cm page width) but with page width less than, let's say, 16-17 cm, you can read those pdf comfortably on almost every 6" reader in landscape mode after cropping/zooming, but to be always able to do that easily you should;
For Kindles install kindlepdfviewer(KOReader) thereon and for Sony PRS, Nook and other rootable android readers install good pdf reader like Orion.
https://www.mobileread.com/forums/sho....php?p=2466450
http://www.youtube.com/watch?v=Ln_GNGhTgZU
If you don't want to install anything on e-reader, you can just easily crop the white borders (with Briss or Pdfscissors) beforehand and then print-to-file such cropped pdf in Adobe Reader (Poster mode 120x90 mm), in about 5-10 minutes for an average book, or use k2pdfopt application to do that in one go.
https://www.mobileread.com/forums/sho...&postcount=440
For A4 pdf's the best solution is bigger e-ink like Kindle DX (with installed kindlepdfviewer), M92, Icarus Excell, Pocketbook 903/912 etc.