The soda contains CO2 that is dissolved CO2 reacts with water
CO2+H2O--->H2CO3
to form carbonic acid
this is a weak acid but it gives protons to water
H2CO3+H2O-->-HCO3+H30+
pH=-lg[H3O+]
[H3O+] is the molar concentration
Therefore the acid present in the solution causes the pH to be lower than 7. It can either be the H2CO3 if it is a carbonated beverage or citric acid from lime juice.