pH has three potential levels. Alkaline, Neutral(7.0) and Acidic. When you inject CO2 into the water you are injecting Carbonic Acid. This causes the pH to move toward the acidic.
kH is basically a buffer. The higher the kH the more buffer you have. The lower the kH the easier it is for acids or alkalines to change the pH. Low kH does make it easier for the CO2 to drop the pH. That is why people with a kH of 2.0 or 3.0 will add baking soda or crushed coral to their water table. To increase the buffer and help to prevent what is called a pH crash.
If you know your kH and your pH there are tables that will tell you what the CO2 is in ppm.
Len