We claim the ornamental design for a portable tissue holder, as shown and described.
FIG. 1 is an exploded perspective view of a portable tissue holder showing our new design; the broken lines depict environmental structure for illustrative purposes only and form no part of the claimed design;
FIG. 2 is a side elevational view thereof, the other three side elevational views being identical thereto; and,
FIG. 3 is a top plan view thereof, the bottom plan view being identical thereto.