EPUB (short for electronic publication) is a free and open e-book standard by the International Digital Publishing Forum (IDPF). Files have the extension . epub. EPUB is designed for reflowable content, meaning that an EPUB reader can optimize text for a particular display device. EPUB also supports fixed-layout content. The format is intended as a single format that publishers and conversion houses can use in-house, as well as for distribution and sale. It supersedes the Open eBook standard.