PDF scripting reference?

I don't think that's a feasible thing to print out, unless you really want to do so. Now of course I'm assuming you're wanting to print it out because I don't see any other reason why you want a PDF version. You'd be looking at printing over 1100 pages worth of documentation at a standard type size.

I don't believe Unity provides a PDF version of their script reference, but they do store an offline version of it, which you can access from Unity by going to:

Help => Scripting Reference

You can probably convert these files to PDF yourself, or just use the HTML files that are included inside the Unity program folder instead.