Hi there, @AshleyWeaver ...
Thank you for sharing the screenshot. PDF files should be fine to upload into Canvas assignment submissions. Can you identify anything within the actual file name that might be causing this error? For example:
Have your students reached out to Canvas Support to see if they can help identify any common threads as to why they are getting this error message?
How do I get help with Canvas as a student?
If students use the "Report a Problem" link (as detailed in this Guide), it is best that they are first looking at the page where the issue is occurring in Canvas. Then, click on the "Report a Problem" link ... which will send some additional metadata along with the ticket so that a Canvas technician can contact the student a.s.a.p.
I hope this will help a bit. Let us know if we can be of additional help here in the Community...thanks!