Preview.app allows PDF files to be combined using drag and drop, but it’s very fiddly, and if you get it wrong, it won’t work. It’s especially tedious when you have many files to combine, for example an expense report where you have separate scans for your receipts, but you need to submit them as a single PDF. http://gotofritz.net/blog/howto/joining-pdf-files-in-os-x-from-the-command-line/ shows that Automator remains useful. You can either make an automator application or folder action that allows PDF files to be combined, or you can use the command line.