Reverse Osmosis is a good technology, but it removes the good with the bad. It takes out Calcium, Magnesium and the essential trace elements along with alkakinity. It doesnt remove the Carbon Dioxide completely, so the pH does tend to drop. This is not a problem, the loss of minerals is.
What I am saying is dont rely on pH. If you are going to drastically purify your water with RO/distillation, you then need to add the minerals back. First check your tap water, most of it is OK and if you use a carbon filter you can remove the chlorine and disinfection byproducts with taking out the minerals.