While Preview’s bookmarks and search features are great for jumping between different pages in a PDF document, sometimes it would be handy to view more than one page of a single document on the screen at the same time. Unfortunately, Preview (like most Mac OS X applications) lets you have just one instance of a file open at a time. But there’s a way around that restriction.
The blue command-line program accepts the parameter -n, which allows it to launch the application again, even if it’s already running. So if you’re currently viewing file.pdf in Preview, enter open -n file.pdf in Terminal. (You’ll need to supply the full and complete path to the file, of course; the easiest way to do that is to type open -n, then drag the file from the Finder to the Terminal command line.)
Another instance of Preview will launch, complete with its own Dock icon, while the PDF document remains open. Unfortunately, some applications (such as iTunes) resist even this technique, so you can’t launch a second iTunes instance – to have multiple iTunes libraries open at the same time, for example.