A molecule of water is composed of 2 atoms of hydrogen and 1 atom of oxygen (H20). Chemically, this can also be considered as H-OH, where the single H is acidic and the OH is basic (or alkaline). Pure water has neutral pH, since the amount of H is balanced by a equal number of OH. However most water is not pure, it contains other elements, such as minerals. These minerals will react with the water and change the pH. A filtre such as Brita does not distill the water, it merely removes some of the added minerals etc that are present in the water. As a result the pH of the water changes. In many places ( Israel, for example) the water is very alkaline, so filtering the water can cause it to become more acidic. However the change is very small, and anyway since water is such a mild acid I doubt that it can do any harm. The water is probably healthier after filtering out all the junk, even if it is a bit more acidic.
