FIG. 1 is a bottom front perspective view of a portable display device showing our new design;

FIG. 2 is a bottom rear perspective view thereof;

FIG. 3 is a front view thereof;

FIG. 4 is a rear view thereof;

FIG. 5 is a top plan view thereof;

FIG. 6 is a left side view thereof;

FIG. 7 is a right side view thereof; and,

FIG. 8 is a bottom plan view thereof.

The shade lines in the Figures show contour and not surface ornamentation.

The broken lines in the figures show portions of the portable display device which form no part of the claimed design.