An EPUB file is a portable digital publication packaged as a single file. Unlike a PDF, which normally preserves a fixed page layout, a common EPUB is reflowable: its text can adapt to a phone, tablet, computer, or dedicated ereader. Readers can often change font size, line spacing, margins, and theme without changing the source book.

What is inside an EPUB?

An EPUB uses a ZIP-based container, but changing the extension to .zip only reveals part of the story. The archive follows a defined publication structure:

META-INF/container.xml points to the package document.
The package document, often an .opf file, contains metadata, a resource manifest, and the spine reading order.
EPUB 3 normally uses an HTML navigation document for the table of contents.
EPUB 2 commonly uses an NCX navigation file.
Chapters are usually XHTML documents accompanied by CSS, images, fonts, SVG, or media.

This web-like structure explains why EPUB works well in a browser, but it also means a reading system must handle the content cautiously. A publication can include scripts, remote images, external fonts, links, and complex markup.

EPUB 2 and EPUB 3

EPUB 3 is the modern standards family and supports richer HTML, accessibility metadata, MathML, SVG, media overlays, and more. Many existing books still use EPUB 2 structures, so practical tools need to understand both package styles.

Support is not all-or-nothing. A simple novel and a scripted fixed-layout textbook can both use the .epub extension while requiring very different reading-system capabilities. That is why EPUB Toolset lists limits separately on each tool page.

What EPUB Toolset does

EPUB Toolset focuses on DRM-free publications that can be handled locally in a modern browser. You can use the EPUB Viewer to inspect a book, correct package information with the EPUB Metadata Editor, change artwork with the EPUB Cover Editor, or extract readable content as Markdown or TXT.

The original file remains on your device. Tools that make changes create a separate download rather than overwriting the source EPUB.

EPUB limitations to remember

The .epub extension does not guarantee that a file is DRM-free, valid, safe, or compatible with every reader. Before publishing, use the official EPUBCheck tool and review the book in the reading systems required by the target distributor.