In Mac OS X 10.5 and later, you can merge two PDF documents together using Preview.

1. Right-click (or control+click) on the first PDF file and select Open With -> Preview.
2. In the Menu Bar at the top of your screen, go to View -> Sidebar. A sidebar pane will appear.
3. Drag the second PDF file into the sidebar window, and it will add them to your PDF document as additional pages.
4. Go to File -> Save.

Your first document will now be a combination of both documents.