Rinse onion in a fine strainer with cold water. Cut peel and outer white membrane from grapefruit and tangerines, following curve of fruit; then cut fruit into 1/2-in. pieces, discarding any seeds.
In a large bowl, gently toss onion, grapefruit, tangerines, avocados, jalapeños, and cilantro to mix. Squeeze lime juice on top and drizzle with olive oil. Season with salt and pepper, add pumpkin seeds, and mix again.
Serve immediately with tortilla chips.