install gs

use command

gs -q -dNOPAUSE -dBATCH -dSAFER -sDEVICE=tiffg4 -sOutputFile=fax_test.tif -f test.pdf

?

/usr/bin/gs -q -sDEVICE=tiffg3 -sPAPERSIZE=a4 -r204×196 -dNOPAUSE -sOutputFile=/var/spool/asterisk/outgoing/mydocument.tif — /var/spool/asterisk/outgoing/mydocument.pdf