We claim the ornamental design for a portable fan, as shown and described.
FIG. 1 is a front elevational view of a portable fan;
FIG. 2 is a rear elevational view thereof;
FIG. 3 is a left side elevational view thereof;
FIG. 4 is a right side elevational view thereof;
FIG. 5 is a top plan view thereof;
FIG. 6 is a bottom plan view thereof;
FIG. 7 is a front/left/top perspective view thereof, wherein the portable fan is in a bent configuration; and,
FIG. 8 is a rear/right/bottom perspective view thereof, wherein the portable fan is in a bent configuration.
The broken lines represent the bounds of the claimed design; the broken lines are for illustrative, environmental purposes only and form no part of the claimed design.