PDF Viewer is a powerful PDF organization and annotation app teachers and students can use. There are several annotation tools available in the app.
To annotate, open a single PDF. Each page can be swiped to (if there are multiple pages present) and annotated individually. Tap the Edit icon in the toolbar to open the annotation tools.
If you're using the Drawing tools, the thickness, color, and overall appearance of the annotations can be changed with the color selection tool (only visible when the pen is selected).
After you're finished annotating, the document can be shared back out to Canvas, Seesaw, or via Apple Classroom. Tap the Share icon to bring up the sharing options.
If students are annotating documents for an assignment, the shared PDF can be sent right to Canvas or Seesaw using the Share menu. The PDF can also be saved as a new document in the Files app.