Is it possible to modify the PDFs?

Yes, modifying the PDF copy of the submission is possible. Unless you are looking for something different. You seem to be searching too but posted it here instead. 

Here are guides you can check for PDF customization:

https://www.jotform.com/help/384-How-to-Customize-PDF-Submissions-Report

https://www.jotform.com/help/457-How-to-Create-a-Customized-PDF-or-Google-Doc-for-Your-Submissions