If you don't even trust the teacher to be able to open a PDF, I'd probably be more concerned about them opening the .docx file with Microsoft Office 97, or whichever archaic version they have always used on their PC, and ending up with a completely broken assignment.
Edit: Sorry I misread your comment, you talked about the process of digitally grading a PDF, I was thinking of the process of simply opening a PDF.
There are many ways to contribute. I actually read an article about that a couple of days ago, maybe it will be of interest to you, too: https://github.com/readme/featured/open-source-non-code-contributions