Sometimes you need to have two views of the same PDF document (e.g. to have a quick access to the list of references in a thesis without the need to scroll to it and then back to the text). However, Preview does not allow a file to be opened twice.
The quickest solution would be to hit Cmd-P and select 'Open PDF in Preview.' You'll have a temporary copy of your PDF where you can select the second view and have both views handy for easy switching.
When you're done, just hit Cancel in the temporary document.
[crarko adds: I tested this, and it works as described. A nice workaround; I looked and didn't find any duplicates of the hint.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20110519204306279