This is an excellent question, as a reliable answer is amazingly hard to find on the Internet. Most advice works for some PDF files, but not for others. The following commands appear to work OK for any PDF file. If you can find PDF files for which these commands fail, please report this.
Ghostscript is the key tool for converting PDF files to FAX compatible TIFF files. You may see other programs being used for this job, but behind the scenes they usually use Ghostscript to do the hard work.
A T.4 1D compressed standard resolution TIFF file can be produced with a command of the form.
gs -q -sDEVICE=tiffg3 -r204x98 -dBATCH -dPDFFitPage -dNOPAUSE -sOutputFile=out.tif in.pdf
A T.4 1D compressed fine resolution TIFF file can be produced with a command of the form.
gs -q -sDEVICE=tiffg3 -r204x196 -dBATCH -dPDFFitPage -dNOPAUSE -sOutputFile=out.tif in.pdf
A T.4 1D compressed super-fine resolution TIFF file can be produced with a command of the form.
gs -q -sDEVICE=tiffg3 -r204x392 -dBATCH -dPDFFitPage -dNOPAUSE -sOutputFile=out.tif in.pdf
A T.4 2D compressed standard resolution TIFF file can be produced with a command of the form.
gs -q -sDEVICE=tiffg32d -r204x98 -dBATCH -dPDFFitPage -dNOPAUSE -sOutputFile=out.tif in.pdf
A T.4 2D compressed fine resolution TIFF file can be produced with a command of the form.
gs -q -sDEVICE=tiffg32d -r204x196 -dBATCH -dPDFFitPage -dNOPAUSE -sOutputFile=out.tif in.pdf
A T.4 2D compressed super-fine resolution TIFF file can be produced with a command of the form.
gs -q -sDEVICE=tiffg32d -r204x392 -dBATCH -dPDFFitPage -dNOPAUSE -sOutputFile=out.tif in.pdf
A T.6 compressed standard resolution TIFF file can be produced with a command of the form.
gs -q -sDEVICE=tiffg4 -r204x98 -dBATCH -dPDFFitPage -dNOPAUSE -sOutputFile=out.tif in.pdf
A T.6 compressed fine resolution TIFF file can be produced with a command of the form.
gs -q -sDEVICE=tiffg4 -r204x196 -dBATCH -dPDFFitPage -dNOPAUSE -sOutputFile=out.tif in.pdf
A T.6 compressed super-fine resolution TIFF file can be produced with a command of the form.
gs -q -sDEVICE=tiffg4 -r204x392 -dBATCH -dPDFFitPage -dNOPAUSE -sOutputFile=out.tif in.pdf
Spandsp can accept T.4 1D, T.4 2D, or T.6 compressed files for transmission, and translate to whatever compression scheme is required during the FAX transfer. Ghostscript is a great piece of software, but it can be a little heavy on CPU requirements. High volume PDF->TIFF conversion is probably not the kind of processing that is wise on a high volume FAX server.