I haven't seen any approach plates on Kindle yet; I downloaded my POH for BE36 but I think it would be difficult to use in the air except in ideal conditions. Reason: as soon as you touch the screen the page turns. I have more than 40 titles on my Kindle so far and like it a lot as a reading machine, instantly text searchable, with built-in dictionaries.
I reformatted a few PDF files to A5 format to be more readable but unformatted A4 format is really too small to be useful.