PDFs are fixed-layout. EPUBs are flowable. On an e-reader, that difference is everything.
Here is which to use for which device, and how to switch.
Where PDF wins
Looks identical everywhere. Preserves images, complex layouts, fonts. Good for textbooks, illustrated guides, scientific papers where layout matters. PDF on a 10-inch iPad is still pleasant.
Where EPUB wins
Reflows text to fit any screen. Adjustable font size and line spacing. Native to Kindle, Kobo and Apple Books. For novels, essays and text-heavy reading, EPUB is the right format.
Best for…
PDF for design-led documents and big screens. EPUB for text-led reading on small screens. If your reader supports both, match the format to the content.
Converting between them
Calibre is the open-source standard for converting EPUB to and from PDF. For PDF-led conversions to text, convert-pdf-to-word plus a manual EPUB step also works.
FAQ
Can Kindle read PDF?
Yes, but reflow is poor. EPUB or AZW3 is much better on Kindle.
Can I convert EPUB to PDF for printing?
Yes, via Calibre or any e-reader's 'export to PDF'.
Which file is smaller?
EPUB is usually smaller for the same text content.
Pick by reader, not by habit. Convert files you actually want to read, not just store.