Actually we can easily and comfortably read most of pdf books on any 6" screen (if we prefer eink screen) because they are usually A5 sized or A4 two-column formated (by using landscape mode with cropped margins or two-column mode).
Any scanned Pdf can also be highlighted with annotations, searched through, scribbled on etc. after quick or detailed OCR-ing (using Abbyy, Acrobat, k2pdfopt or any other OCRing app there).
If our 6" reader has poor pdf capabilities or is slow in landscape mode, then we can use k2pdfopt to quickly adjust pdf for our reader or tablet beforehand.
k2pdfopt can also reflow one-column A4 pdf scans for 6" readers, but 10" eink readers or lcd tablets are a lot better solution for one-column A4 pdf (landscape, with or without margins cropped).
Those who want to read pdfs, djvus and epubs on Kindles, Kobos or Pocketbooks should immediately install KOReader(KPV for older Kindles) thereon, though.
KOReader on Kobo H2O, from 21:00 min.
https://www.youtube.com/watch?featur...zVAMpDQ#t=1263
KOReader on Paperwhite 1
https://www.youtube.com/watch?v=4QMATVc6ZeA
https://www.mobileread.com/forums/sho...d.php?t=254659
http://www.willus.com/k2pdfopt/