In this video from middiablo we learn how to combine multiple PDF files into one document on a Mac. Navigate to where the PDF files are in the finder and command click on them and then open with preview. Make sure the thumbnails button is checked at the bottom right. In order to select them all, click on one page, hold down shift and then click on the last page. It will select every page. Now click and drag and you can put the pages anywhere you want. Now go to File and Save. An even easier way is through automator. Type get specified finder items. Save as plug-in for the finder. Now in finder using work flow right click on the PDF items, then More, then Automator, then Combine PDF. You PDF's are now combined.