I'll expand my requirements a bit. I probably should have done this at the start.
Lightweight. I want to carry this in a backpack while I ride.
Cheaper than an IPad. Because there is every chance that I'll break it tossing the bag around.
I don't require all the IPad functionality and would prefer not to pay for it.
Enough memory on board (not a card for security unless it is encrypted) for about 100 pdf documents at screen resolution.