According this source, a study by Dionex, a part of Thermo Fisher Scientific, found that decaf coffee is less acidic than regular coffee.
That being said, all coffee causes the stomach to produce acid and the scientific results of the effect of both types of coffee on this are inconclusive. Some studies have found that patients drinking decaf have less stomach acid issues and vice versa.
According to this source, the nature of the beans used to make decaf coffee are higher in acidity in any case, because these beans are used due to their ability to retain flavor after treatment.
All in all, decaffeinated coffee beans have less acid in them than their counterparts, but because the choice beans for decaffeination are already high in acid, this may make most decaf coffees have a higher acidic content in general when compared to other coffees.