The pH of soda pop is mostly determined by the dissolved carbon dioxide which produces carbonic acid. The pH of pop is around 2 or 3.
Different parts of the human body have different pH values. The stomach is about 1, while the normal pH of human blood is about 7.4. It would seem very unlikely that the pH of the body would change much because of smoking. The body has an elaborate system to regulate the pH of the blood and hold it at about 7.4. This is called buffering and our bodies have all sorts of systems to buffer the blood pH. Even if smoking did make a difference, the body would compensate and get the pH back to normal. The only indirect way in which smoking might affect body pH is through damage to the lungs. There are many respiratory diseases that wreak havoc with the body's pH regulation system, and some of these could be caused by a lifetime of smoking. Most of these cause the blood to become more acidic. This of course is compensated in various ways because so many systems within the body require a steady pH of 7.4. But this compensation can lead to other problems in other parts of the body that are compensating, i.e. the kidneys. A good reference source on this is pH of the Blood by Michael J. Bookallil.