Put the pineapple juice in a small (non-aluminum) saucepan. Bring to a boil. Lower heat to medium and cook until the juice is reduced to 1 cup. This takes quite some time. Remove from heat and cool. Stir in soy sauce. Place the salmon in a shallow casserole dish and pour the pineapple-soy sauce mixture over the fish. Add a bit of black pepper. Refrigerate for 2 hours, turning the fish after the first hour.
Place in a very large nonstick saute pan over medium heat until hot. Add the salmon and cook for 3 minutes. Using a spatula, carefully turn the fish over and cook for 3-4 minutes, until just cooked through. Do not overcook. Meanwhile put the remaining pineapple-soy mixture into a small skillet.Cook over high heat until reduced by half, about 5 minutes. Using a pastry brush, brush the top of the salmon with some of the reduced marinade. Remove the fish from the pan and serve.