Chlorine dioxide remains a gas in solution and doesn't have a PH associated with it.
When you bubble chlorine dioxide through water, the PH of the water remains the same.
When the PH of sodium chlorite is lowered by adding acid to it, chlorous acid is formed. It is the chlorous acid that accounts for the lower PH. Chlorine dioxide is a gas and has no PH associated with it.
I have been working with a dentist on a very strong mouthwash. It is made from using HCl to activate sodium chlorite. In addition to making sure that the chlorine dioxide concentration is kept below levels that do damage to the gum tissue and enamel on the teeth, we also have to be mindful of the PH of the solution. I have been using Baking Soda to buffer the solution with excellent results.