Which documents are proof of Indian citizenship, if not Aadhaar, passport or Voter ID?

No single document is proof of citizenship for all Indians, rather, this depends on how a person acquired it under Citizenship Act, 1955. A combination of documents establishing a person's birth date and place, parents' citizenship or a certificate of naturalisation may be required. By law, those born between January 26, 1950 and July 1, 1987 are citizens by birth.