public static function delete_feedback_document($assignment, $userid, $attemptnumber) {

        $assignment = self::get_assignment_from_param($assignment);

        if (!$assignment->can_view_submission($userid)) {
            print_error('nopermission');
        }
        if (!$assignment->can_grade()) {
            print_error('nopermission');
        }

        $grade = $assignment->get_user_grade($userid, true, $attemptnumber);

        $contextid = $assignment->get_context()->id;
        $component = 'assignfeedback_editpdf';
        $filearea = self::FINAL_PDF_FILEAREA;
        $itemid = $grade->id;

        $fs = get_file_storage();
        return $fs->delete_area_files($contextid, $component, $filearea, $itemid);
    }