Most of our books come in two formats. They contain the same text, but they behave differently on different screens.
EPUB: for phones and e-readers
An EPUB reflows. The text adapts to your screen size, and you can change the font and text size. It is the best choice for phones, e-readers and small tablets. Apple Books and Google Play Books open EPUB files, and Kindle accepts them through Amazon's Send to Kindle service.
PDF: for large screens and printing
A PDF keeps a fixed page, exactly as designed. It looks best on a laptop, a large tablet or on paper. On a phone you will need to zoom and scroll sideways.
The short version
Reading on something small? Choose EPUB. Reading on something big, or printing? Choose PDF. You get both, so you can switch whenever you like.