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EPUB

EPUB is a free, open ebook standard that is compatible with almost all ebook readers currently available, with the notable exception of the Kindle (Kindle users should choose Mobipocket instead). This is also the format that's usually used for ebook lending by libraries.

EPUB supports reflowable text, and does not have any concept of pages / page numbering. Some readers attempt to add page numbering based on their screen size, the size of the EPUB document etc, but these methods are not standardised, and are unlikely to be the same on different devices.