pH is a measure of the acidity of water. We humans drink a variety of different beverages with varying pH ranges, from highly acidic orange and apple juice (3.3 pH), to mildly acidic milk and coffee (6 pH). Liquids that have a 7.0 pH are considered neutral and anything above that is alkaline.
Water can be acidic or alkaline, depending on the source. In my view the ideal drinking water is a pH of about 7.4.
Reverse osmosis water purifiers will reduce the pH of drinking water. Because reverse osmosis removes the minerals in water the water will then react with carbon dioxide upon exposure to air to form carbolic acids, thus lowering the pH. The resultant pH will depend on the initial water chemistry. It cannot be predicted.
Reverse osmosis drinking water will tend to taste crisp, some people would say metallic. I add a calcite filter to my systems to raise the pH and add a taste resembling spring water.
Some people object to drinking reverse osmosis water because it does remove minerals but in many instances reverse osmosis is the healthiest choice. Please see my article on Minerals in Water.