Combine, re-order and save.
Have you wondered if it was possible to merge PDF files together into one file without downloading third-party software? Well, turns out you can -- and it's really simple, too! You can use Preview, the PDF viewer app that's included with your Mac, to move pages from one PDF document to another. The Apple support page walks you through the steps, and the Tuts+ screencast (Youtube clip) gives a quick demo.