The ornamental design for a portablearmrestexpander, as shown and described.
FIG. 1 is a bottom plan view of a portable armrest expander showing my new design;
FIG. 2 is a front elevational view thereof;
FIG. 3 is a left side elevational view thereof;
FIG. 4 is a front perspective view thereof;
FIG. 5 is a rear perspective view thereof;
FIG. 6 is a rear elevational view thereof;
FIG. 7 is a right side elevational view thereof;
FIG. 8 is a top plan view thereof;
FIG. 9 is a perspective view thereof showing an unused state; and,
FIG. 10 is a perspective view thereof showing a use state.
The broken line showing of the chair and person is for the purpose of illustrating the environmental structure and forms no part of the claimed design.