Distilled water tends to test between 7.0 and 6.8 on the pH scale because water exposed to the open air absorbs carbon dioxide and becomes more acidic.
Distilled water is often exposed to carbon dioxide in the air at some point during the distilling process. Pure water is 7.0 and completely neutral, but distilled water sources can get as low as 5.5 on the pH scale. This level of acidity is not generally a threat, but it can dissolve light metals like copper. This is why copper is not a good material for conveying water through pipes; if a light metal is used in conveying water, if that water becomes even as acidic as a pH of 5.5, pieces of the copper may end up getting dissolved by the water, becoming a part of it. Consumers could then drink the water and develop metal poisoning, which can be deadly. Distilled water at a perfect 7.0 pH is generally hard to achieve, and for the most part, it must be done in a lab. According to doctors like Andrew Weil, distilled water is generally as close to a neutral pH as people can get, and it is perfectly safe to drink.