In order to accomplish this you need to assume that the person sending the "fax" has the ability to generate a PDF. If that is the case, then the "fax" is unnessary.
You can sign up for wab fax services that send an image file (I believe in tiff format) that can be read using their reader. Once the file is open, you can print it to the distiller and have a PDF.
Good luck
Aloha,
cg