eBook file formats
11 formats in this category.
eBook formats hold long-form text that reflows to fit any screen. Most are zipped bundles of HTML and images, with layout controlled by the reading app.
Reflowable books and their formats
eBook formats hold long-form text that reflows to fit any screen and font size, unlike a fixed-layout PDF. EPUB is the open standard, a ZIP bundle of HTML and images supported by almost every reader except Amazon's. Kindle devices use Amazon's own line: older MOBI and newer AZW3 (KF8), which are based on the Mobipocket structure.
Because EPUB is essentially web content in a wrapper, it adapts beautifully to phones and e-readers, and converting between EPUB and Kindle formats is straightforward with free tools such as Calibre.
DRM and compatibility
The main reason an eBook won't open is not the format but digital rights management. A DRM-protected book is locked to a particular account or app, so even a supported reader will refuse it until it is authorised. DRM-free EPUB and MOBI files, by contrast, open in any compatible reader and convert freely.