Our students filled out pdf files with their information and uploaded them. When I go to grading, all their annotations are missing from the view. If I open the pdf uploaded by the students, I can see their info both in Foxit Reader, Acrobat Reader and Acrobat DC.

Steps to reproduce the problem:

1. Make a course with an activity that accepts submissions in pdf format.
2. Enrol the course, set yourself both to Teacher and Student.
3. Get a pdf file that's not protected so you can write on it in any pdf viewer (I tried Foxit 9.7.1 and the latest Acrobat DC).
4. Type any text on the pdf and save it.
5. Upload it as a submission to your activity.
6. Go to grading, find your submission, click Points.
7. Moodle converts the uploaded pdf into an other file and presents a view. The student-made annotations are missing from the view.
8. Still on the Grading page there is a link to the uploaded pdf file. Click on it.
9. Your browser downloads the pdf file. Open it in any external pdf viewer. The annotations will be there.