The ornamental design for a portable displaydevice, as shown and described.
FIG. 1 is a perspective view of a portable display device;
FIG. 2 is a front elevation view of the portable display device of FIG. 1;
FIG. 3 is a rear elevation view of the portable display device of FIG. 1;
FIG. 4 is a left side elevation view of the portable display device of FIG. 1;
FIG. 5 is a right side elevation view of the portable display device of FIG. 1;
FIG. 6 is a top plan view of the portable display device of FIG. 1;
FIG. 7 is a bottom plan view of the portable display device of FIG. 1;
FIG. 8 is a perspective view of the portable display device of FIG. 1 illustrating an example in which an image is displayed on a display screen of the portable display device;
FIG. 9 is a perspective view of the portable display device of FIG. 1 illustrating another example in which an image is displayed on a display screen of the portable display device; and,
FIG. 10 is an elevation view illustrating the portable display device of FIG. 1 in use.
Broken lines shown in the drawings illustrate portions of the portable display device and form no part of the claimed design.