The ornamental design for the portable fax machine, as shown and described.
FIG. 1 is a front view of the portable fax machine.
FIG. 2 is a rear view of the claimed design.
FIG. 3 is a left side view of the claimed design.
FIG. 4 is a right side view of the claimed design.
FIG. 5 is a top view of the claimed design.
FIG. 6 is a bottom view of the claimed design; and,
FIG. 7 is an isometric view of the claimed design, shown with a sheet of paper in broken lines, which is environmental matter and forms no part of the claimed design.