# What is remote resources in EPUB?

Source: https://epubcc.com/glossary/remote-resources/ (updated 2026-11-04)

A remote resource is a file an EPUB loads from the web instead of carrying it inside the ZIP. EPUB 3.3 allows it only for audio, video, fonts and resources fetched by scripts; everything else, including images and stylesheets, must be inside the container.

## Why it matters when you convert a PDF to EPUB

An e-book is read offline, on planes and on e-ink devices without a browser, so anything loaded from the web may simply be missing. PDFs that link to images or fonts online, or HTML copied from a website, are the usual way remote references sneak into an EPUB.

A content document that does use an allowed remote resource must say so with the remote-resources property on its manifest item. EPUBCheck reports a forbidden remote reference as RSC-006, a missing property as OPF-014 and a declared property with nothing remote as OPF-018.

How EpubCC handles it: everything the engine writes is inside the container, so its EPUBs never reference remote resources. Links in the text to websites are ordinary hyperlinks, which are allowed.