In order to have a web site generate PDFs, or perform any other PDF operation site server has to have some kind of PDF tool/libary. And the server script that processes the web pages has to be integrated with the tool.
You can buy the official Adobe PDF Library from Datalogics. They also have lots of other tools for performing PDF operations on a server. But there is a huge variety of 3rd Party PDF libraries and tools as well, such as Active PDF, Foxit, PDFLib, iText, PDFBox, Appligent, and many more.