You can form a droplet on the leaf that acts as a magnifying glass, focusing the light and burning the area beneath. It's not common indoors, but it does happen.
Your pH reading on the rain water doesn't matter too much because rain water is REALLY REALLY 'soft'; it is not bad to use at all as long as you add the same dose of CalMag Plus that you would use on distilled or RO water. Check the pH AFTER you add nutrients with rain water, or any other water for that matter.

You are lucky- you have GH 3 part so you have a bottle of 'micro'. You can add about a tablespoon of that to your spray bottle and do light foliar feedings to restore color and lost nutrients to the upper leaves. A drop of dish soap helps it not make beads of water that can burn the leaf.