Authors preparing an ebook for the first time often assume a "file" is a file, and that whatever software produced their manuscript can simply export something a retailer will accept. EPUB complicates that assumption slightly, because it is not a picture of a page in the way a PDF is. It is closer to a small, self-contained website, packaged so that reading apps and devices can display it consistently.
This guide explains what EPUB actually is, how it differs from formats authors are more familiar with, and what practical implications that has for text, images, navigation and accessibility. For how format choice fits into wider publishing decisions, see [should I publish paperback, hardcover or ebook?](/resources/book-design/should-i-publish-paperback-hardcover-or-ebook)
EPUB explained
EPUB (short for "electronic publication") is an open, widely adopted ebook file format maintained under standards published by the W3C. Structurally, an EPUB file is a compressed package containing HTML files for the text, CSS style sheets controlling appearance, image files, a table of contents document, and metadata describing the title, author and other bibliographic details.
Because it is built on web technologies, an EPUB behaves more like a small website than a printed page. A reading app opens the package, reads the HTML and CSS, and renders the content according to both the file’s styling and the settings the reader has chosen on their own device.
Reflowable EPUB
Reflowable is the default and most common EPUB layout style. In a reflowable book, the text is not locked to fixed page breaks. Instead, the reading app decides, in real time, how much text fits on a "page" given the reader’s chosen font size, line spacing, margins and screen dimensions. Turn a phone from portrait to landscape, and the text reflows to fit the new shape of the screen.
This is what allows a visually impaired reader to enlarge text substantially, or a reader on a small phone screen to still comfortably read a novel formatted for a large tablet. It is a genuine usability advantage that print and fixed formats cannot offer in the same way.
Fixed-layout EPUB
Fixed-layout EPUB is a different variant of the same underlying standard, used when the precise position of text and images on a page matters and cannot be left to the reading app to decide. Instead of flowing text, each page is defined with elements placed at specific coordinates, similar in spirit to a PDF, but still built on EPUB’s underlying HTML and CSS structure.
Fixed-layout EPUB generally does not support the same reflow benefits as standard EPUB, and font-size adjustment by the reader is typically limited or unavailable. It trades some accessibility flexibility for exact visual control.
EPUB versus PDF
A PDF is fundamentally a fixed representation of a page, designed to look identical regardless of the device displaying it. That consistency is useful for print-ready files, but it is a poor fit for small screens, because a PDF page sized for a paperback book will often require pinching, zooming and horizontal scrolling to read comfortably on a phone.
EPUB, by contrast, is built specifically to adapt. This is why most general ebook retail and library platforms prefer or require EPUB (or a platform-specific equivalent) for standard fiction and non-fiction, and treat PDF as better suited to print-style documents, reports or heavily designed layouts rather than reflowable reading.
EPUB versus Kindle delivery
Amazon Kindle devices and apps do not display EPUB files directly. When an author uploads a manuscript to Amazon KDP, KDP converts the source file into its own Kindle-compatible format as part of the publishing process. Authors sometimes assume this means EPUB is irrelevant to Kindle publishing, but many workflows still start from, or pass through, an EPUB-like structure before conversion, and EPUB remains the standard delivery format for most other retailers and libraries.
Because Amazon’s accepted source formats and conversion behaviour are set and updated by Amazon, check Amazon KDP’s current documentation before assuming exactly what file type to upload at the time you publish.
Text
Body text in an EPUB is marked up as HTML, styled with CSS rather than fixed positioning. This means formatting choices such as headings, italics, drop caps or scene-break symbols need to be applied using styles that a reading app can interpret, rather than relying on the visual layout tricks common in word processors or print design software.
Images
Images are embedded as separate files within the EPUB package and referenced from the HTML, similar to a webpage. Image resolution and file size matter: oversized images can bloat file size and slow loading, while undersized images can look poor on high-resolution screens. Illustrated and heavily visual books often need more careful image handling than plain text fiction.
Table of contents
EPUB supports a structured table of contents that reading apps use to build their own navigation menus, separate from any table of contents printed within the visible text. A well-built EPUB usually includes both: a navigable structural TOC for the reading app, and, where appropriate, a conventional TOC page the reader can see while reading.
Accessibility
Accessibility features, such as alternative text for images and a logical reading order for screen readers, are supported within the EPUB standard rather than being bolted on afterwards. Building these in from the start, rather than trying to retrofit them, generally produces a better result for readers using assistive technology.
Fonts
EPUB files can embed specific fonts, but many reading apps allow readers to override the publisher’s chosen font entirely in favour of their own preferred typeface. This is a deliberate accessibility feature of reflowable EPUB, and it means a book’s typography in EPUB form is generally a suggestion to the reading app rather than an absolute guarantee, unlike a fixed print layout.
Links
External hyperlinks, such as a link to an author’s website or another book, work in EPUB much as they do on the web, provided the reading app supports opening external links. This is one of several ways an ebook edition can be given interactive elements that a print edition cannot.
Validation
Because EPUB is a structured standard, files can be checked against that standard using validation tools before submission to a retailer or distributor. An invalid EPUB may still open in some reading apps while failing or displaying incorrectly in others, which is why validating a file rather than only "eyeballing" it in one app is good practice.
Metadata
EPUB files carry their own embedded metadata, including title, author, language and identifiers, separate from whatever metadata a retailer’s upload form collects. Keeping these consistent avoids situations where a file’s internal metadata disagrees with what is displayed on a retailer’s product page.
When fixed layout may be needed
Fixed-layout EPUB tends to suit books where the visual arrangement of text and image is part of the content itself, such as heavily illustrated non-fiction or picture-book-style titles, where a reflowable layout would break the intended pairing of text and image. It is a deliberate trade-off, not a universally "better" version of EPUB.
Children’s and illustrated books
Children’s picture books and other tightly designed illustrated titles are the category where the reflowable-versus-fixed decision comes up most often, because reflowable EPUB can separate text from the specific image it was written to sit beside. Authors working in this space often need to weigh reflowable EPUB’s accessibility benefits against fixed layout’s visual control, on a book-by-book basis.
Testing on devices and apps
Because different reading apps and devices render EPUB somewhat differently, testing a finished file across more than one app, and ideally more than one type of device, before publishing is a sensible final check. A file that looks fine in one reading app can occasionally reveal formatting problems in another.
Common mistakes
- Treating EPUB as "just a PDF for ebooks". The two formats behave fundamentally differently on-screen, and files built for one do not automatically work well as the other.
- Assuming Kindle files and EPUB files are identical. Amazon converts uploaded files into its own Kindle format; check current KDP documentation before assuming a raw EPUB is what is required.
- Fighting reflow with print-style formatting tricks. Manual line breaks, forced spacing and fixed indents designed for a printed page often render oddly once the text is allowed to reflow.
- Skipping validation. A file that opens in one app is not proof that it is a valid, well-formed EPUB across all reading systems.
- Ignoring accessibility basics. Missing alt text and disorganised reading order make a book harder to use for readers relying on assistive technology, and are simple to address at production time.
Frequently asked questions
Is an EPUB the same as a PDF?
No. A PDF is a fixed page image, while a standard EPUB is reflowable and adapts to the reader’s device and settings. They behave very differently on small screens.
Do I need to upload an EPUB to Amazon KDP?
Amazon KDP converts uploaded manuscripts into its own Kindle-compatible format. Check Amazon KDP’s current documentation for accepted source file types at the time you publish.
Can a reader change the font in an EPUB?
In most reflowable EPUB files, yes. Many reading apps let readers override the publisher’s font, text size and line spacing, which is a deliberate accessibility feature of the format.
When would I need fixed-layout EPUB instead of reflowable?
Fixed layout suits books where exact text and image placement matters, such as some illustrated or picture-book titles, at the cost of some reflow-based accessibility benefits.
How can I check if my EPUB file is valid?
EPUB validation tools can check a file against the published EPUB standard before you submit it to a retailer or distributor, which can catch problems that only show up in some reading apps.