Place cucumber slices in a colander; sprinkle with salt. Toss well. Drain 1 hour. Place cucumber slices on several layers of paper towels (or a clean dish towel); cover with additional paper towels. Let stand 5 minutes, pressing down occasionally. Rinse and pat dry.
Combine vinegar, water, sugar, and pepper in a saucepan. Bring to a boil. Reduce heat; simmer 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.