Focusing on the iPad and iPhone, EPUB is only one of the two formats Apple's iBooks application supports. The other is Portable Document Format (PDF). PDF is an open standard for document exchange. PDF was created by Adobe Systems in 1993. PDF is used for representing two-dimensional (2D) documents in a manner independent of the application software, hardware, and operating system. Each PDF file encapsulates a complete description of a fixed-layout 2D document that includes the text, fonts, images, and 2D vector graphics, which compose the documents. Lately, 3D drawings can also be embedded in PDF documents.
Image: PDF on an iPhone (Click image to enlarge.)
PDF is designed to reproduce page images. PDF text traditionally could not be re-flowed to fit the screen width or size. As a result PDF files designed for printing on standard paper sizes are less easily viewed on screens with limited size or resolution, such as those found on mobile phones and PDAs. Adobe has addressed this by adding a re-flow facility to its Acrobat Reader software, but for this to work the document must be marked for re-flowing at creation, which means that existing PDF documents will not benefit unless they are tagged and resaved.
PDF documents can be read on an iPad. That is great for access to legacy and transitional documents. But, if you are hoping to have your eBook reach a larger audience, consider EPUB.
Image: EPUB on an iPhone (Click image to enlarge.)
A great source for much of this post and recommended reading is Elizabeth Castro's great book, EPUB Straight to the Point: Creating ebooks for the Apple iPad and other ereaders, published by Peachpit Press in both print and electronic editions.