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.