The ornamental Design for a poster display device, as shown.
FIG. 1 is an isometric view from the front and to one side of the new design;
FIG. 2 is a front elevational view thereof;
FIG. 3 is a side elevational view thereof; and,
FIG. 4 is a top plan view thereof.
In the drawings, the broken lines shown encircling the shaded poster display device define the boundary of the claimed design and form no part of the claim; the broken lines below the device depict a municipal sign post and form no part of the claimed design. It is understood that the supporting structure may also be a street sign, traffic sign, traffic light standard, street light standard, utility pole, transit signpost or any other municipal signpost structure.