There are many ways to share a PDF could you give us the route that your teachers are using for the most part? Could you share an example? For an example if the teacher is utilizing the storage provided by canvas they should be able to whitelist the root domain (yourschool.instructure.com) and all additional partitions should come through ex: https://yourschool.instructure.com/users/0000/files/00000/preview?verifier=BOSn7ZKyi5bBhfkswuzoKdoZz... But if the teachers are using a third party like Microsoft's 365 stuff or Google Docs or drive to host PDFs that is an entirely different question.
All in all Whitelisting becomes a daily task with many ways to share content there will always be a something to correct so students and teachers can function. Blacklisting might be more feasible and with many automatically generated list (for gaming, porn, malicious, propaganda, etc )it would be less of a hassle.
This discussion post is outdated and has been archived. Please use the Community question forums and official documentation for the most current and accurate information.