Hi @jmerriam,
I think the best way to do this would be to create an Annotation-type online assignment for the students to complete. You'd upload a template document, and then students would use Canvas tools to add text/draw on it, basically like you can usually do in SpeedGrader with student files.
The alternate way to do this would be to create the online assignment, and in the description of the assignment, provide a link to the file that you uploaded and direct students to use that as their starting point and then they'd upload the completed version as their submission.
Hope one of these two methods will work for you!
-Chris