Preheat oven to 300°.
Coat a 15 x 10-inch jelly-roll pan or other large rimmed baking pan with cooking spray. Place popcorn in a large metal or glass bowl lightly coated with cooking spray.
Combine syrup, butter, salt, and pepper in a saucepan over medium heat. Bring to a boil, stirring just until combined. Cook, without stirring, for 2 minutes. Pour syrup mixture over popcorn in a steady stream, stirring to coat.
Spread popcorn mixture in an even layer into prepared pan. Bake at 300° for 15 minutes. Remove from oven, and cool completely in pan.