EPUB
Also known as: epub file, ebook format, electronic publication
EPUB is the open standard format for ebooks. It is a ZIP package of HTML, CSS, and images, and its text reflows to fit any screen size — unlike a PDF, which keeps a fixed page layout. Most e-readers except Amazon Kindle support it natively.
- Open ebook standard — a ZIP of HTML, CSS, and images
- Text reflows to any screen, unlike fixed-layout PDF
- Supported by most e-readers except Amazon Kindle
How EPUB works
An .epub file is essentially a small website zipped together: chapters as HTML, styling as CSS, plus fonts and images. A reader app re-flows the text to match your font size and screen, which is why EPUB suits novels and long-form reading.
This reflowable nature is the main difference from PDF: a PDF fixes the page, while EPUB adapts. EPUBs are usually compact because text compresses well, with cover art and embedded illustrations adding most of the weight.
EPUB and storage
A text-only ebook is often just a few hundred kilobytes; illustrated or image-heavy titles run larger. On iPhone and iPad, the Books app stores EPUBs and can offload them to iCloud, keeping local copies of only what you are reading.
If a library of downloaded books is taking up space, removing finished titles — they re-download from your purchases or library — reclaims it.