Hi @fetzer_gail_c ,
Thanks for your question! If you cancel out of submitting the assignment the PDF will be removed. Alternately, if you click the "Add Another File" link you will be able to select a new file and remove the PDF you previously added.
I hope this helps!
Great question! I have done what @Gail Fetzer described. Also, if you have already submitted the assignment you can click on submission details on the top right of the assignment page and add a comment that says something like, "Please disregard my previous PDF attachment. This is the updated document I'd like to submit." Then, click attach file and re-submit. Hope this helps.