I have found if you create and send a standard Word Document through the Exetel email2fax gateway it works fine and sends a great quality fax.

Problems occur when sending scanned documents as the quality of the received fax can be atrocious.

These are the steps I used under Linux to provide a decent quality of PDF to send as an attachment via the Exetel email2fax gateway. (This is from trial and error so there may be improvements still to be had).

1. Using XSane
2. If it's multiple pages set xsane to scan to "multipage"
3. Select 300DPI "Line Art" (Not colour or grayscale)
4. Scan your pages and save as PDF.

Problems: If the resulting PDF is over about 4MB it will expand (by about 134%) and get rejected by the email2fax gateway, so check it before you send. To get a smaller PDF I saved the multipage xsane output as Postscript and then converted to PDF using ps2pdf which made a smaller PDF size (and it reduces quality so check it before sending an important fax!)