On a Mac, you can just use Preview to "Save as" or when you Print, select PDF. But if you're unlucky enough to use a PC then your options are more limited.
To get this program, visit CutePDF and download your free version (you don't need the Pro version and if it prompts you, uncheck the option to install a browser toolbar, you don't want that either).
PDF Printer. It's pretty cool and can convert documents, photos, and web pages, but it's not free ($5.99 from the App Store).