Step 1: Open the course with the file you want
Step 2: Click Modules on the left hand side
Step 3: Select the desired file
Step 4: Download the file
Step 5: Save the file to your downloads folder (By clicking and dragging)
Step 6: Launch Squid
Step 7: Click the plus in the bottom, right hand corner
Step 8: Click "Import PDF"
Step 9: Find the file
Step 10: Start editing!