I claim the ornamental design for a paper clip holder, as shown and described.
FIG. 1 is a perspective view of a paper clip holder showing my new design, with the cover in the closed position; and,
FIG. 2 is a perspective view of the paper clip holder with the cover in a party open position, showing paper clips inside, the paper clips being shown for illustrative purposes and forming no part of the design.
The bottom of the paper clip holder is flat and unadorned.