Preheat oven to 400 degrees. Place puff pastry on a lightly floured parchment-lined baking sheet and gently stretch into a 9 1/2-inch square. With a paring knife, score pastry 3/4 inch from edges, creating a border. Freeze 10 minutes.
Stir together raspberries, sugar, lemon juice, flour, and salt. Lightly mash some berries to release juices and let stand 5 minutes. Very gently stir in pears. Spread evenly within border of pastry. Brush border with egg white and sprinkle with sugar.
Bake 15 minutes. Reduce temperature to 375 degrees and bake until juices are bubbling and crust is deep golden brown, about 30 minutes more (tent crust with foil if overbrowning). Dip a pastry brush in juices and glaze fruit. Let cool 15 minutes before serving (or let sit at room temperature, up to 8 hours).
One serving of this crisp, sweet tart has just 100 calories and 2 grams of fat.