Once the PDF has been generated, you can't change the way its content is printed. Well... you
could, but you'd have to design another document template that displays the PDF and masks out the text.
If you have PlanetPress Workflow, then it becomes easy. Use a process in which a branch prints the document, while another branch creates the PDF. Both would use the same template. And inside the template, the text object you want to hide would have a condition that looks like this:
This will ensure that the text only appears on the PDF.