ePub or PDF: which e-book format to choose
The choice comes down to one question: does the layout of the page carry meaning?
If it does, use PDF. If it does not, ePub will give the reader a better time on almost every device.
What each format actually does
A PDF describes a fixed page. Every character sits at a known position, and it will sit there on a phone, a laptop and a printer alike. That is the format’s whole purpose, and it is why it won in offices and in print production.
An ePub file describes content, not pages. The text reflows to whatever screen it lands on, and the reader picks the font and the size. There is no page five, because how much text fits on a screen depends on the screen.
Where PDF is the right answer
Anything designed. A magazine, a brochure, a report with charts and sidebars, a form. Move the text around and the meaning goes with it.
Anything where position is the information. Sheet music, a mathematical proof laid out in columns, a technical drawing, a table that only works at a certain width.
Anything to be printed. PDF is a print format that also gets read on screens.
Comics and illustrated books. ePub can carry fixed layout, but a PDF is simpler and every device opens it.
Where ePub is the right answer
Prose. A novel, a biography, an essay collection. Nothing is lost by reflowing it, and a lot is gained.
Anything read on a phone. This is the decisive case. A PDF sized for A4 on a phone screen means pinching and dragging on every page; an ePub simply fits.
Anything a reader needs to adapt. Larger text, a different typeface, a dark background, a screen reader. ePub makes all of that the reader’s choice. A PDF’s text is fixed at the size the designer chose.
What each one costs you
PDF’s weakness is small screens, and it is a serious one. A reader who has to pinch and scroll will usually give up before the end.
ePub’s weakness is that you give up control. You cannot promise that a diagram sits beside the paragraph that explains it, or that a table holds its shape. If that matters, it matters a lot.
In practice
Most devices open both, so this is rarely a dead end. Calibre converts between them, though the conversion is only comfortable in one direction: an ePub becomes a decent PDF, while a PDF made of fixed pages becomes an ePub full of broken line breaks, because the structure a reflowing format needs was never in the file.
If you are producing something and can offer only one, ask who reads it and on what. A report read at a desk is a PDF. A book read on a train is an ePub.