There are a few things you need to set -
- Go to Site administration -> Plugins -> Activity modules -> Assignment -> Assignment settings. Under this, set the first setting for 'Feedback plugin' to 'Annotate PDF'
- Next, go to Site administration -> Plugins -> Activity modules -> Assignment -> Feedback plugins -> Annotate PDF. Check whether the first setting under this, 'Enabled by default' is checked or not. If unchecked, please put a check mark against it and make it enabled for use.
- Go to your assignment. Open up assignment settings, under the section 'Feedback types', put a check against 'Annotate PDF'.
This should enable the annotator and hence, you will be able to view PDF documents for annotation purpose while grading assignment submissions in Moodle.
However, there are some issues with the annotator as discussed in the given links -