Edit your php config to disable zlib Go to the Annotate PDF admin settings page Verify there is a warning about zlib being required Create an assignment and upload a pdf as a student Mark the assignment and verify no option to "Launch PDF editor" appears. Edit your php config and enable zlib again Visit the same admin page and verify there is no warning Grade the assignment and verify there is now a "Launch PDF editor" button in the grading page. OPTIONAL, if you are a developer and have access to the DB, verify that the assignfeedback_editpdf/zlibenabled isn't ever written to DB, aka: select * from mdl_config_plugins where plugin = 'assignfeedback_editpdf';