See the PDF Export link at the top of the Libraries page.

So do you paste the code in the library that you want to use into your own project?

To use a library you first need to call :

1. import libraryname;

This will let your program know about all what's done inside the library.
So, for PDF export you have to call

1. import processing.pdf.*;

Then you can access the library classes, methods and all the magic provided by the library.
In order to achieve some PDF export, you have several situations (do you need screen display ? do you want to export one single frame or several ?...). That's what you can see at the page phil.lo suggested.

Then, you have to adapt your code to make it look like the structure provided at the pdf export examples.

To make it work, you just need to add the lines needed.