Hi @MillicentMabija,
Welcome to the global Canvas Community!
It sounds like you are trying to upload (submit) a PDF file to an assignment that you have been assigned. Is that correct?
If it is correct, please go to the How do I upload a file as an assignment submission in Canvas? - Instructure Community - 274 support article (there is a short video) that explains how you can upload (submit) your PDF file to the assignment.
If that is not correct, please provide us with more information so that we can help you.
You may also want to consider reaching out to your school's Canvas team or IT Department for help.
-Doug
