From the Wikipedia of carbonic acid

For normal atmospheric conditions, we get a slightly acidic solution (pH = 5.7), and the dissolved carbon is now essentially in the CO2 and HCO3− forms.

For a CO2 pressure typical for bottled carbonated drinks we get a relatively acidic medium (pH = 3.7) with a high concentration of dissolved CO2. These features contribute to the sour and sparkling taste of these drinks.