The best documentation can only be viewed by running Pd. Click on the "Browser..." menu item on the "Help" menu when you run Pd, and you will get an interface for navigating through the included documentation.

You can browse the Pd documentation online on this site by going to the Documentation section.

You can also see Miller Puckette's manual on his site.

And for a Wikipedia-style collection of information, check out the Pdpedia