The solution is to do everything digitally, scribbling your signature with your finger, a stylus, or a mouse cursor, and pasting it on the PDF. There are multiple different ways to go about it, so ...

Don't have a printer and scanner? You still can easily sign a PDF file, no matter the operating system you're running.

Fortunately, Apple has added the ability to electronically sign a PDF document using Preview, a program that comes preinstalled on every Mac running OS X Lion or later.