The ornamental design for a pocket handkerchief, as shown and described.
FIG. 1 is a front elevational view of a pocket handkerchief showing my new design, the undisclosed rear is not part of the claimed design;
FIG. 2 is a front elevational view of a second embodiment of a pocket handkerchief showing my new design, the undisclosed rear is not part of the claimed design; and,
FIG. 3 is a front elevational view of a third embodiment of a pocket handkerchief showing my new design, the undisclosed rear is not part of the claimed design.