I often have to combine mutiple PDF files into a single file for convenient storage, and I found a really easy way to do this using an Automator script with my iMac. This web site explains how in a few simple steps you can create such a script and have it available as a “Service” in a right-click popup context menu. So to use it, you select which PDF file you want to combine then do a right-click and select the “Combine PDF Files” service in the popup menu. Easy!
Note, that this only works for the Apple Mac OS system. Below is what the script looks like in the Automator GUI: