If you use a Windows based machine there are other options for annotating a PDF beyond the built in annotation tool in HFCOnline/Moodle. Drawboard is one program that at the time I write this is on sale for S5.99 (It is usually twice that.)
The video below shows some of the things you can do with Drawboard. One way to use it somewhat efficiently (this idea stolen from my depatment chairperson) is to ask students to turn in assignments as a single PDF in HFCOnline. HFCOnline has a way for you to download as a .zip file all student response files. You can unzip the file, then grade and comment using Drawboard.
Then you can go to grade in HFCOnline, select grade for that assignment, enter each student’s score and upload their file. It is not as convenient as the built in annotation tool but Drawboard itself is easier to use than the editor in HFCOnline especially if you have a Windows based tablet.