The ornamental design for portable printer, as shown and described.
FIG. 1 is a front view of a portable printer illustrating our new design;
FIG. 2 is a rear view thereof;
FIG. 3 is a top plan view thereof;
FIG. 4 is a bottom view thereof;
FIG. 5 is a left side view thereof;
FIG. 6 is a right side view thereof; and,
FIG. 7 is a top perspective view thereof.
The broken lines immediately adjacent to the claim, encompassing broken line regions, depict the bounds of the claimed design and form no part of the claimed design, while all other broken lines are directed to environment. The broken lines form no part of the claimed design.