As discussed on Mac Geek Gab 370, the Preview application in Mac OS X is an extremely powerful tool. One of its less-obvious features is the ability to combine and re-order multiple PDFs into a single document, something that was once the purview of only expensive third-party software. 

Open your first document in Preview. At the top of the window you’ll notice four buttons next to the search field. Select the second button, which will add the Preview sidebar to the left of the PDF document.

In the sidebar, you can now drag individual pages (or groups of pages if you use Shift-click or Command-click to select multiple pages) and re-order the document as you please. You can even selectively delete pages by selecting the page and pressing the delete key on your keyboard.

Once you’ve rearranged the document to your satisfaction, simply save it before closing Preview to give your changes permanence.

To combine multiple PDFs into a single document, open a second PDF in a separate Preview window and click the button in the toolbar to bring up the sidebar, as discussed above. Next, select the pages from the second document that you want to add to the first (use Command-A to select all if you want the entire document merged).

Once selected, simply drag the pages from the sidebar in the second document and drop them into their desired relevant position in the first document’s sidebar.

There’s one important note: at the top of the sidebar in each document is the name of the document with a disclosure triangle to the left and a horizontal line below. When dragging your pages from the second document to the first, make sure to drop them below this line. If you drop them above the line, it will merely “move” the second PDF into the first PDF’s window, allowing you to view both documents in the same Preview window. It will not combine the documents. 

Just as with rearranging a single document, once you’ve combined the pages you desire, simply save the changes and your first document will now contain the pages you chose to add. 

While this example focuses on two PDFs, the practice will work the same with a virtually unlimited number of documents, allowing you to fully customize and combine all your PDF data into one handy file.