Preheat oven to 400 degrees F and line a baking sheet with parchment paper. Set aside.
In a large bowl, mix together pumpkin puree, condensed milk, egg, and cinnamon.
Add one layer of filo dough on lined baking sheet. Brush a thin layer of butter on its surface. Once buttered, add another sheet of filo dough on top.
Continue for the remaining filo sheets. Do not butter the top and final layer.
Pour pumpkin mixture in the center of the filo dough (widthwise). Sprinkle with chopped pistachio.
Roll the sides and ends over on top of the filling, securing with butter and slightly pressing down. It should look like a flat log.
Carefully turn the pastry over so the seams are facing side down.
Bake for 20 minutes and allow to cool. Sprinkle powdered sugar and cut to serve.