PDF Mergy is a free tool for combining PDFs. The service does not require you to register unless you want to select files from your Google Drive account. To merge files just select them from your desktop or your Google Drive account and PDF Mergy does the rest. When your merged files are ready you can download them. The video below shows you how the service works (the video does not have sound).
There aren’t too many times that I’ve had to merge PDFs, but the few times that I have PDF Mergy would have made it a heck of a lot easier for me.PDF Mergy is available as a Chrome Web App.
One safety note about PDF Mergy; I would not use it for PDFs that have sensitive student information in them.