The Trijicon Walther P99/PPQ/PPQ M2 Heavy Duty Night Sight Set will not fit a Walther PDP. The Walther PDP has replaceable sights designed to co-witness with the specific red dot you plan on mounting to your gun. These sights can be purchased through Walther. However, the AmeriGlo sights located at the link below will replace the stock sights on a Walther PDP.
https://www.opticsplanet.com/s/walther-pdp-sight?_iv_categories=tritium-fiber-optic-night-sights&_iv_brands=ameriglo-2976144788