This does not look like powdery mildew, but it does resemble edema, a condition of cellular damage that can happen when plants are over-watered or kept too humid (so the leaf cannot expel excess water). Are the peppers being allowed to dry out somewhat between waterings? (Where the soil feels somewhat dry to the touch about 1 to 2 inches down before they are watered again.) Is the humidity level high, where it's regularly above about 70% or so? Using a small fan to circulate air around the foliage may help. Nothing can heal leaf damage, so the current symptoms won't go away even if the problem is solved, but new growth should not develop the edema symptoms if the conditions change enough to discourage it from recurring.

Miri