It isnt merely ph adjustment... You want some amount of minerals in water for your health, plants, and taste. Changing the PH isnt the concern in most cases, its just part of the result.
RO makes water more acidic. if water was somewhat acidic to start with, it can get more acidic or become corrosive.
RO doesn't output pure water. if you want pure water you slap DI filter after RO membrane.
Ph change is one part of the result, not the goal. The goal is water purification.
those for example https://www.freshwatersystems.com/collections/specialty-cart...
or those https://www.freshwatersystems.com/collections/filters-media?...