Slice zucchini into 1/16-inch-thick rounds using a mandoline. Place on a clean work surface and sprinkle with salt. Let sit for 10 minutes.
Dab zucchini slices with paper towels to remove as much moisture as you can.
Preheat the oven to 220 degrees F (105 degrees C).
Place zucchini in a large bowl. Add oil and gently stir to coat. Add grated Parmesan cheese and ranch powder and gently stir to coat.
Spray 2 large baking sheets with nonstick cooking spray. Arrange zucchini slices on the sheets, making sure none are overlapping.
Bake in the preheated oven for 40 minutes. Carefully flip the slices and continue baking for 30 minutes, or until desired crispiness is reached. Transfer to a paper towel-lined plate.