Government of India has deliver the aadhar card service to unite India and for unique India. Aadhaar service is free service for all , for the residents of India who are living currently in India and including NRI too. The document required for Aadhar card to any NRI is the as the same as any other resident of India.
As the same as Indian resident if any NRI wanted to apply for aadhar card then he should be present at the aadhar cantre can be said be the enrolment centre of India near by hi location.
NRI also have to submit the same document and follow the same documents as the normal Indian resident submits. He will also asked for Iris scanner and finger print.