This is such a slow process. It's just a PDF file, do I really need to load Acrobat Reader? Do I have to get Excel fired up just to take a quick look at some of the values in a spreadsheet? Quicklook saves all that and is about as easy to use as it get.
All you need to do is open the Finder and select a file, then hit the Space bar. Quicklook loads a preview nearly instantly that shows you the contents of most files. Since I've been moving a lot of files between machines lately this is a great way to look at a file really quickly.
You can even select a group of files (Command-Click) and then hit Space and you will have the option of viewing an index sheet of your files so you can scan through them quickly. It really is a fantastic little feature.
If you haven't used Quicklook yet you should take a quick look at it.
Sorry about that last one, I had to.