The ornamental design for pen stand, as shown and described.
FIG. 1 is a perspective view of pen stand showing my new design;
FIG. 2 is a front elevational view thereof;
FIG. 3 is a rear elevational view thereof;
FIG. 4 is a left elevational view thereof;
FIG. 5 is a right elevational view thereof;
FIG. 6 is a top view thereof; and
FIG. 7 is a bottom view thereof; and,
FIG. 8 is a front elevational view of the two parts of the pen stand separated.
The broken lines showing features inside the top openings in FIGS. 1, 5 and 6 and a portion of one side of the pen stand in FIG. 4 as well as the objects shown between the separated parts of the pen stand in FIG. 8 are included for the purpose of illustrating environment and form no part of the claimed design.