EPA secondary standards for ...

EPA secondary standards for drinking water pertain to contaminants that are not considered health threatening but can have undesirable effects.  See https://www.epa.gov/dwstandardsregulations/secondary-drinking-water-standards-guidance-nuisance-chemicals, which says that low pH can cause a bitter metallic taste and corrosion of metal water pipes and fixtures. Other sources show that many soft drinks have pH in the range of your collected rainwater, and the acidity of orange juice is even lower.