Pdf Doucment Created Scanner Fax Fails Invalid Data

I have Vista Ultimate and a new USRobotics PCI FaxModem. WFS faxing works fine if I compose the content of the fax in the WFS New Fax editor window and then click send. But if I try to insert an attachment - a PDF or TIFF document created with my Microtek scanner - the fax fails with an "Invalid Data" error message. Is my scanner creating a file that WFS cannot render? Or is there some other known issue with faxing scanned documents?If WFS is choking on my scanner's output, how do I know which file formats/types WFS can render and which scanners output acceptable formats?