biggest problem with pdfs on e-readers is that they are usually formatted for a partiuclar "page size" which is at least double the average e-reader screen size. The upshot of this is the text areas are far too wide, which means to fit them left-to-right on the screen, they shrink to an uncomfortable size.

Short answer for .pdf: more pages, narrow or zero side margins, fewer words per line. Imagine printing the PDF to the size of an e-reader screen and you'll be on the right track.

The _best_ answer is to provide a .epub format. then there are no issues.