Ok, in this case the easiest solution would be to prepare some sort of "thank you" page with the link to PDF. Then, configure the Form to redirect to this page after the form submission, so that after submitting a form the user will be redirected to the page with PDF link.
However, if the user will open this "thank you" page via direct URL (without completing the form) he will still be able to see and download the PDF. If this is not acceptable, then some custom logic would need to be implemented.