When students submit assignment files to Moodle assignments you can use the built-in viewer and editor to annotate them. You can add comments, highlight, or draw on student work from within Moodle.
Visit the assignment and click the Grade option
You should now see a student submission.
Using the toolbar you can:
If you are happy with your annotations, click Save. To start over click Reset.
You can click the arrows in the Change user textbox to navigate to the next student submission, search for a student submission by their name, or click the filter to quickly view and jump to submissions