Old fashioned letter -- good idea. I think I'll try that.
They should definitely know they bought a pdf, as the entire listing states several times that that's what it is (as does the title), and also the shipping notification that I sent says it was sent to their email from my email. In addition, every email I've sent has said they're receiving the pattern files, so if they're able to receive email at the address on file, they've been informed at least 6 times now that they are pdf files.
If they don't know they're receiving pdf files at this point, no additional words I could add would clarify that, unfortunately.