There are two issues here.
The Kindle DX is big enough for Jepp-size (A5) approach plates 1:1.
The free "AIP" plates were mostly designed for A4 and when reduced to the A5 size (at which Jepp, Aerad etc plates are published, and usually printed) they are a bit too small to read. Some of the text is especially small. I did a lot of messing around with this a while ago and concluded that 1024 pixels (along the long axis of the plate) is the minimum at which the AIP plates are legible - reading glasses or not.
Jepps are readable fine at 800 pixels, so the usual old 800x600 tablets worked fine.
The Kindle is a lot faster than most e-book readers, so when somebody produces a decent file browser, and does some sort of directory structure for it (it does run unix after all) it will be a winner.
In the meantime, "proper computers" will rule the roost for rapid PDF display.