Overview for E-Readers

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EPUB is ane-book file format with the extension .epub that can be downloaded and read on devices like smartphones, tablets, computers, or e-readers. It is a free and open standard published by the International  Digital Publishing Forum, (IDPF). The term is short for electronic publication and is sometimes styled ePub. EPUB became an official standard of the IDPF in September 2007, superseding the older Open eBook standard. The Book Industry Study Group endorses EPUB 3 as the format of choice for packaging content and has stated that the global book publishing industry should rally around a single standard. EPUB is the most widely supported vendor-independent XML-based (as opposed to PDF) e-book format; that is, it is supported by the largest number of hardware readers.

What is a PDF?

PDF is referred to as an e-book but is really a photograph of a page. Officially named  Portable Document Format , it was invented by Adobe as a way of distributing print documents digitally. Publishers have been using the same method to transfer print-ready files to commercial printers for decades.
The PDF retains its format no matter the size of the screen that displays it. It will always be an duplicate of the paper document that it represents. Thus, a smaller screen size means very small word size, since the number of words on any given page does not change.