problems using group4 compression

In command line I found two ways to convert jpg to pdf with group4 compression:
First ( with two commands ):
$ convert desktop.jpg desktop.pbm
$ convert -monochrome -compress group4 desktop.pbm desktop-mono-grp4-2.pdf

Second ( with one single command and best compression ):
$ convert -thereshold 80% -compress group4 desktop.jpg desktop-mono-grp4-2.pdf