There are many apps available to do this outside of Canvas. GoodReader, Notability, and PDF Expert are just a few that come to mind.

With Canvas Student, if an instructor includes a PDF in the assignment's instructions/details, students are given the option to annotate the item right inside Canvas Student and then submit it! It saves things in-progress in case the student can't finish in a single sitting, too. How do I add annotations to a submission in the Student app on my iOS device?‌