[thow_away_34564]

I think the answer is in that PDFs are used to convey 90%+ static content offline in a way where you could open up 10-year-old PDF and it will still look exactly like it did the day it was created. Because of this is it the system of record for reports in business, government, academia, and other organizations.

The other use case are fillable forms that are legally binding. In larger business or the government where you have your identity tied to a private key, you can sign a PDF with it.