I guess you do not understand. I have an USB fax modem which has worked fine until now with 10.9.5. One goes to print then hits the PDF button. There should be a drop down menuitem FAX PDF. It is missing. It does not matter what printer is selected.
I figured it out with your help. All I needed to do was go to the Printers & Scanners panel, select the + for adding then the FAX tab. It shows the USB modem. I just select it and I got my FAX back.That is one detail that you did not include in your first post. Was there software that was included with your Fax modem? Are you able to ‘see’ it listed in the Printers & Scanners panel?