Some tasks need a really simple, straightforward, no-nonsense app to get the job done. Combining PDF files is one of those tasks, and Gelatin is the app to do it.
Preview can do a similar thing, but it's certainly not as straightforward as Gelatin. The app offers a window to drag-and-drop the files you want combined together onto. Select them in finder in the order you want them melded and drop them onto Gelatin. The app will take care of the rest spitting out a PDF with all the files you've dropped onto it joined end-to-end.
You can take the file and edit it up with Preview, email it off or save it. Gelatin is free for a limited time and is worth a download for the shear ease of use. If you ever need to combine PDF files, making Gelatin part of your workflow could save you time and effort.