I already know the solution, but I thought I'd post it here in case anyone else has the same issue.
My marketing department asked me to embed a custom font into my PDFs generated with mPDF. I followed all of the instructions per the mPDF documentation, but I couldn't get it to work. After some debugging, an old fact about font licensing popped up in my head.
Not all fonts have web licenses or are embeddable. The safest source for "open source" fonts, with free and commercial use, is google fonts.
I was able to successfully add my new font after downloading a font from google, uploading the .ttf to the font directory, and following the instructions found at https://mpdf.github.io/fonts-languages/fonts-in-mpdf-7-x.html.
It would be a great idea to update the wiki to include that tidbit in case other people have issues adding custom fonts too.