[markom]

8" e-ink reader should be good enough for about A5 sized scanned pdf in portraite mode zoomed without margins (the majority of paper books there are about that size), or in landscape mode and for A4 multi-column pdfs.

Front light is the main reason for me to buy 8" e-ink though.

9.7" readers like Kindle Dx(with installed KPV reader thereon) or M92 are very good for any A4 sized pdf zoomed in landscape mode, with or without margins.

Flipping through pdf pages at about one second per e-ink screen is probably quick enough for the majority of users there, but a lot of users are probably still not happy with its handwriting and highlighting speed and capabilities, compared to tablets.