The widely used Portable Document Format (PDF) for page-based documents was introduced by Adobe in 1993. It is based in large part on the Postscript page description language for modern printers, which had been developed in the late 1970s/early 1980s. In 2008 the specification for PDFs was made available by Adobe as a royalty free open standard and adopted by the International Standards Organization (ISO32000-1).
This PDF Wiki provides practical information based on the use of PDFs and PDF tools over the past decade.
One of the most important features of PDF documents is they are defined by a PAGE BASED model - this describes how individual pages in the document are made up, in terms of the text, the fonts used, graphical objects, interactive elements and possibly other features associated with the page. This PAGE BASED model means that when you look at a PDF page on-screen or on printed output, it should always look the same and as specified by the designer. This is completely different from formats such as ePUB and HTML, which are not page based - they are effectively a linear stream of items, one after another, with limited "layout" elements (ePUB3 and HTML5 have improved on this of course, but they are still very much flexible, flowable formats). The two approaches have been designed independently, with a major aim of formats like ePUB being to allow the text to be the dominant element, re-sizable and re-flowable, ignoring the page concept and focusing on the size and orientation of the device on which it is viewed. ePUB and its variants and versions is the most widely used format for reading eBooks documents on mobile devices, including of course Amazon Kindle, Nook and other specialized ebook reader devices.