I found a solution to my problem and thought I'd post about it in case anybody else comes across this thread in the future. The solution is called jPDFtweak, which is an open source freeware app that you can easily find by doing a google search for "pdf tweak."
It was a little confusing to figure out at first (for example, you select the option to add page numbers to your .pdf in the "Watermark" tab, not in the "Page Numbers" tab, as you might think), but once I did it worked well for my specific task.
My searching also revealed an app in the Mac Appstore called Proview PDF Editor, that is supposed to insert page numbers as well. It costs $4.99. I didn't try that one out since my first find did the trick.