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

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.

This is a rather specific question, but can you cast audio from arbitrary apps to WiFi speakers from your Pixel? Similar to airplay on iOS (if that's what it is called)?

That's exactly what it is. I haven't looked into it too much, but as far as I know it's main advantage is simplifying the setup process, which in turn reduces the chances of a misconfigured VPN.