Just tried a couple of A5 approach plates in PDF format on a Nook bookreader by Barnes & Noble (a 6-inch 800x600 e-ink screen). A full page is readable with a lot of strain; however, when zoomed to fit width, it is quite clear. Obviously, this kind of screen only gets better in bright sunlight. The zoom function is not available in the stock PDF viewer, but Nook runs Android and is easily hackable. In fact, it also has a miniPCI slot (occupied by the modem in the 3G version), which could in principle accommodate a GPS receiver...