You will get bigger better pumpkins if you prevent powdery mildew so the vine can photosynthesize to its maximum potential. Since it already has spread substantially and probably has spores on all of your leaves, you might as well just let it take it's course, but next year use a sulfur based fungicide in powder or liquid form (I prefer mixing powder with water because the liquid is so expensive and use it so fast and powder is hard to apply). Leaving the PM is like letting a cloud cover set in until it is so thick that it is like night time all the time and the leaf is useless or negatively hurting the plant by consuming extra nutrients to feed the parasite.