Preheat oven to 350°F. Line a baking sheet with parchment paper and set aside. In a large bowl, whisk together the flour, pumpkin pie spice, baking powder and salt. In the bowl of an electric mixer, beat the eggs, brown sugar, pumpkin and vanilla until well combined. Add the flour mixture, one cup at a time, until a thick dough forms. Form dough into a large loaf on the prepared baking sheet. Bake for 30 minutes or until loaf just starts to crisp on the outside. Remove from oven and let cool for about 10 minutes. Using a serrated knife, cut the loaf into 1/2-inch-thick slices and place each slice flat on the parchment-lined baking sheet. Bake the slices for about 45 minutes, flipping the biscotti every 15 minutes, until it is dried out and crisp. Let cool completely on the pan. Store in an airtight container at room temperature for up to 5 days.