In a medium bowl stir together flour, baking powder, cinnamon, baking soda, and salt; set aside.
In a large mixing bowl beat together eggs, pumpkin, sugar, and oil on medium speed. Add the flour mixture; beat until well combined. If desired, stir in chopped pecans.
Spread batter into an ungreased 15x10x1-inch baking pan.
Bake in a 350° F oven for 25 to 30 minutes or until the center comes out clean. Cool completely in pan on a wire rack.
In a medium mixing bowl beat together cream cheese, butter, and vanilla until fluffy. Gradually add sifted powdered sugar, beating until smooth.
Frost pumpkin bars. If desired, top with pecan halves. Cut into squares. Store, covered, in refrigerator up to 3 days.