I claim the ornamental design for a paper towel holder, as shown and described.
FIG. 1 is a perspective view of a paper towel holder of the present design;
FIG. 2 is a top view of the paper towel holder of FIG. 1;
FIG. 3 is a front view of the paper towel holder of FIG. 1;
FIG. 4 is a right side view of the paper towel holder of FIG. 1; and,
FIG. 5 is left side view of the paper towel holder of FIG. 1.
The broken (dashed) lines shown in FIGS. 1 through 5 are included only for the purpose of illustrating environmental elements and form no part of the claimed design.