PDF (Portable Document Format) is one of the most widely used formats for storing and transferring both text and image data. Some of its main advantages include being compatible with any operating system, containing formatting that does not change from one viewer to another and offering great quality at accessible size.
PDF is widely used for uploading online content, and for this purpose in particular it can be optimized via linearization. This basically allows you to download pages as you read them. Or in other words, spend significantly less time between the moment you click on a link to a PDF file and the moment you actually get to read it.