If you regularly use PDF documents, I recommend PDF Download: stops them loading in the browser and asks you what you want to do. DownThemAll is good, and if you're going to get AdBlock Plus, also get the Filterset.G Updater.
Lastly, if you use NoScript, and you're generally safe in your browsng habits, configure it to allow "Base 2nd level domains" by default. This means that the site you browse to can run scripts, but 3rd-party scripts are blocked. The result is that most websites work straight away, but tracking scripts etc. don't.