The Ornamental Design for a poster display device, as shown and described.
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.
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.