The good and the bad thing about tagging documents on the Mac is that there are a bunch of different ways to do it.
If you want to get into tagging, I always recommend that you use software that uses the OpenMeta standard. That way, different applications can all access the same tags.
A free and easy way to get started tagging documents is TagIt by Ironic Software, the company that developened OpenMeta in the first place. This video shows how to do it.
This video on tagging PDFs on the Mac is part of a series of quick videos on paperless tips and topics. View more in the series here.