How EPUB Works
EPUB is the open standard format for ebooks, and under the hood it is essentially a website packaged as a book. An EPUB file is a ZIP archive containing XHTML content documents (the chapters, written in the same markup as web pages), CSS stylesheets for formatting, images, fonts, and a set of metadata files. An OPF package document ties it together with a manifest listing every resource and a spine defining the reading order. So an ebook reader opening an EPUB is really rendering a small, self-contained web publication.
Building on web technology gives EPUB its defining feature: reflowable text. Because the content is XHTML and CSS rather than fixed pages, the text adapts to any screen and any font size. The reader can change the typeface, enlarge the text for comfort, switch to dark mode, or read on a phone or a large tablet, and the book reflows to fit, repaginating on the fly. This is the crucial difference from a PDF: a PDF keeps a fixed page layout, while an EPUB rearranges itself around the reader's preferences, which is exactly what you want for novels and long-form reading.
EPUB is maintained as an open standard (by the W3C), which is why it is supported across most e-readers and reading apps, Apple Books, Kobo, Google Play Books, and many others, rather than being tied to one company. The notable exception historically was Amazon Kindle, which used its own formats, though Kindle now also accepts EPUB. This open, widely-supported nature is a big part of why EPUB is considered the standard ebook format.
EPUB vs PDF vs Kindle Formats
The clearest contrast is with PDF. PDF fixes the page layout, ideal for documents that must look identical and print exactly, but cramped and awkward to read on a phone, where you must zoom and pan. EPUB reflows, making it far more comfortable for reading books on varied screens. As a rule: PDF for fixed documents, EPUB for actual reading. Against Amazon's Kindle formats (MOBI, AZW, KFX), EPUB is the open equivalent; converting EPUB to a Kindle format (or now sending EPUB directly) is how books move into the Kindle ecosystem.
Because EPUB is reflowable, it is less suited to content where exact layout matters, heavily designed pages, complex textbooks, comics, or anything where the position of every element is fixed. For those, a fixed-layout format (or fixed-layout EPUB) or PDF is more appropriate. For standard prose books, EPUB is the better reading experience.
When to Use EPUB
EPUB is the right format for ebooks meant to be read: novels, non-fiction, and any long-form text where the reader benefits from adjustable fonts, sizes, and screen sizes. It is the format to publish to for broad e-reader compatibility, and the one to convert to when you want a document to be comfortable to read on a phone or tablet rather than a fixed page you must zoom around. For fixed, print-exact documents, choose PDF instead.
Limitations
EPUB's limitations follow from its reflowable nature. It does not guarantee a fixed layout, so designs that depend on exact element positions can shift, which is why richly-designed material often stays in PDF or uses fixed-layout EPUB. Rendering can also vary slightly between reading apps, since each interprets the CSS a little differently, much like web browsers. And historically, Amazon's Kindle ecosystem required conversion. None of these affect its core strength: for readable, adaptable long-form text, EPUB is the open standard that works across the e-reading world.
