I am given the option to download a US Letter or A4 formatted PDF - Which fits the iPad screen better?
Letter. A4 is slightly longer and narrower than US Letter, and would result in wider borders at the side to fit a full page vertically. US Letter won't fit perfectly, but the aspect ratio of it's dimensions are a much closer fit to the iPads 4:3 (or 3:4 in portrait) screen and the slimmer borders top and bottom would be less noticeable than the A4 equivalent:
Millimetres Inches Width Length Width Length A4 210.0 297.0 8.26 11.69 (3:4.24 APPROX)
or 6% too long when scaled to fit the shortest edge, resulting in vertical borders when scaled to fit the longest edge
Letter 215.9 279.4 8.50 11.00 (3:3.88 APPROX)
or 3% too short when scaled to fit the shortest edge, resulting in horizontal borders when scaled to fit the longest edge