Hi Oliver,
Sorry, don't know of way to do that. Nearest possibility I can think of would be to give them an editable copy of the file for use in LibreOffice. When they complete their work, they export it as a PDF, which they then submit for marking.
It has been a while since I had students try to, "scribble all over" something for submission. Results were VERY disappointing. Scribbling with a mouse just does NOT work very well without a LOT of practice. If you are lucky enough to have a bunch of "good" tablets with pens available, the results are much better. The best I ever manged was with a M$ Surface Pro 3 using Justin Hunt's PoodLL Online submission setup for white board. The Surface Pro was the best setup I ever used for grading on the PDF's, too. I could legibly write simple comments directly with the pen without having to stop marking and create a text box to type in.