Now, check that in the response you see among other data listed the pdf annotated file, you can download that file appending ?token=USERTOKEN to the file path, and check that the file is downloaded correctly
For the older Moodle versions the only way you can test it is running the unit tests
Create an assignment (require one file) with the PDF annotation feedback tool enabled, see ( https://docs.moodle.org/30/en/Using_Assignment#Annotating_PDF_files for the required settings)
As a student submit the assignment (submit a pdf file), as a teacher review the submission adding some annotations in the pdf file.
As admin, enable "Mobile services": Plugins ► Web Services ► Mobile
Create a Token for the student
Click on Site administration ► Plugins ► Web services ► Manage tokens
Next, you can do a CURL REST call simulating a WS client with one of the student users.
You need to replace the wstoken, assignid (with the instance id, not the cmid) and the URL of your moodle instance
curl 'http://localhost/m/stable_master/webservice/rest/server.php?moodlewsrestformat=json' --data 'assignid=32&wsfunction=mod_assign_get_submission_status&wstoken=ffbe3a3002f235bf9d01fd9369e10b66' | python -m "json.tool"
Now, check that in the response you see among other data listed the pdf annotated file, you can download that file appending ?token=USERTOKEN to the file path, and check that the file is downloaded correctly
For the older Moodle versions the only way you can test it is running the unit tests