Preheat the oven to 400°F (200°C). Line a baking sheet with parchment paper.
On a lightly floured surface, unfold the thawed puff pastry sheets. If needed, gently roll to smooth seams. Cut into 8 equal rectangles (4 per sheet).
Spoon about 1–2 tablespoons of pizza sauce into the center of each rectangle, leaving a 1/2-inch border all around.
Top with shredded mozzarella, a few pepperoni slices (about 1–2 per pocket), and a light sprinkle of Parmesan, Italian seasoning, garlic powder, and (optional) red pepper flakes.
Fold the pastry over the filling to form a pocket. Press edges to seal, then crimp firmly with a fork so the filling doesn’t leak.
Transfer pockets to the prepared baking sheet. In a small bowl, beat the egg with 1 teaspoon water. Brush the tops with egg wash for a glossy, golden finish.
Cut 2 small slits on top of each pocket to vent steam. Bake for 18–22 minutes, until puffed and deeply golden.
Cool for 5 minutes before serving (filling will be very hot). Serve with extra warmed pizza sauce for dipping. Optional: brush with melted garlic butter (melt 2 tablespoons butter with a pinch of garlic powder and Italian seasoning) right before serving.