It was the standard for scanners until PDF seemed to dominate the scene.
Probably because it's much easier (for average users with few tools and skills) to print a PDF than to print any sort of non-page-based (e.g., image) file format and have the resulting sheet of paper match the scanned sheet of paper in terms of scale, orientation, position -- assuming both sheets are the same dimensions. Essentially using the file as an intermediary for physical copying of standard paper documents.
The student just download the app, and it fix the orientation, rotation, bad light, contrast, and many other horrible things that a jpg may have. In particular the orientation and ordering multiple sheets. Also, Moodle has a little more support for pdf than jpg [2].
I don't know how many three letter agencies are reading the stream, but I'm happy that many three letter agencies operative now have a better formation in algebra and calculus.
[1] https://www.camscanner.com/
[2] It depends on how many optional packages your sysadmin installed.
(not sure if the documents in OP had several pages, but if you've scanned/photographed a multi-page document, PDF is not that bad of a solution)