Ended up sending it off via US mail. Hopefully, it'll arrive safely in Austin and be opened before the new year.
If you don't mind sharing, what operating system and browser combo do you use?
Edit: Just tried a test attachment from my home machine. Windows 10/Firefox browser. PDF attached no problem. I'll try my work machine tomorrow and see if still errors. Either the issue is resolved, OR something else is at play.
RSD did tell me they have issues with iOS platforms and cell phone submissions, but I'm not using either.