From 1st of July 2017, Aadhaar number becomes a mandate for tax filings in India. As per the Finance Act 2017, tax payers have to put in their Aadhaar ID or enrolment number for filing income tax returns (ITR). It has also been made mandatory for all new PAN applications with effect from 1st July’17.
However this rule only applies for permanent residents of India (resided in India for a total of 182 days or more in 12 months prior to the application) and does not hold true for NRI’s.
For more details on Aadhaar application please see https://uidai.gov.in/