In Mathematics and other science subjects, assignments usually come with model solutions which are released to students after the due date, and to markers beforehand. The "Solution sheet" plugin allows teachers to upload these model solutions as PDF (or other) files directly inside the assignment activity.
Solutions are initially hidden to students, and the teacher can release them either manually, or at a specified time after the due date. They can also be made unavailable after a configurable date.Plugins DB: https://moodle.org/plugins/assignfeedback_solutionsheet
Moodle Docs: https://docs.moodle.org/32/en/Solution_sheet
The plugin works with Moodle 3.2 or later.