Combining PDF Files
1. Find the document you wish to be the first page of your new, combined document. Two-finger click on the doc or control-click on the doc and select "duplicate." This will preserve the original document.
2. Two-finger click or control-click on the document once more and select "Open With--> Preview"
3. After the file opens in Preview, click on the "Edit" button on the menu and then choose "Insert-->Page from File."
4. Select the file you wish to combine. You can select multiple files by using the shift, or command key while selecting. Then click open.
5. You can then rearrange the pages on the left-hand side by clicking and dragging them in the order you wish them to be. Save the new file when you are done!
You can also Drag/drop files directly into the Thumbnail area to add pages....
If you don't see the thumbnail area, you can click the button in the top-left corner of the screen and select "Thumbnails"