The epub format is pretty much ubiquitous, and many readers (including iBooks) support it. As far as I know all reader apps allow you to choose your own font size but I think not with PDFs, because the fonts are defined inside the pdf. An aspect of the ipad that you might find useful, then, is Zoom, which, with a triple finger double tap will let you zoom the page and use two fingers to scroll around it.
for things like messaging, calendar, notes and so on, you can set your own font size inside Settings --> Accessibility. This is where you'll find Zoom as well.
Good luck
