FPDF is a PHP class. It enables you to create PDF documents
The class is pretty simple to use and can be downloaded from www.fpdf.org
There are manuals and more information available there on exactly how to use it.
The great thing about it is that there are lots of functions that are available to you out the box. This means that you don't need to understand how the functions work and what goes on behind the scenes, you just need to learn how to use the functions.