Place the cucumber slices in a colander; sprinkle with salt. Toss well. Drain 1 hour. Place cucumber slices on several layers of paper towels; cover with additional paper towels. Let stand 5 minutes, pressing down occasionally. Rinse and pat dry.
Combine vinegar, water, sugar, and pepper in a small saucepan. Bring to a boil. Reduce heat; cook until reduced to 1/3 cup (about 10 minutes). Remove vinegar reduction from heat; cool. Stir in onion. Combine cucumbers and vinegar reduction in a medium bowl; toss well. Sprinkle with peanuts.