Select your course from the "Courses" tab at the top of your Schoology account page. Once you are looking at your course page, click on the "Updates" tab on the left sidebar. This tab has updates containing announcements from your teachers, classmates, and pages students would like to discuss with each other!
You will see a text field in the Updates page.
Attach your file by clicking on the "file" icon at the bottom left of the text field, and then select your PDF to upload.
Click "Post" and you're good to go!