This is possibly a duplicate of:
https://tracker.moodle.org/browse/MDL-45969

1. Create an assignment, with attempts reopened manually

2. Student hands in pdf.

3. Teacher annotates it, gives a grade, and saves.

4a. If a new attempt is opened, then the student cannot open the annotated pdf by clicking the link "View annotated pdf": the response is "Cannot open the PDF. The file may be corrupt, or in an unsupported format.", but if he clicks the file beside the pdf logo, whose name includes his name, then the annotated pdf is downloaded correctly.

4b. If no new attempt is opened, then the annotated pdf opens as expected. (But personally I don't understand the advantage of this view.)