As mentioned, Indian, Bangladeshi and Maldivian don't need a pre-applied visa to enter Bhutan. But all nationals need to bear passports with a minimum of 6 months' validity before the travel date. Especially for Indians, they're free to enter Bhutan by air or land because of the free movement treaty between Bhutan and India.
Any of the following documents are applicable upon entry:
1.Indian Passport
2.A Voter ID card with photograph
3.Identification Slip issued by the Indian Consulate Office in Phuentsholing upon presenting proof of Indian citizenship.
For more information about traveling to Bhutan with Indian Passport, please feel free to contact Asia Odyssey Travel and we are happy to help with your Bhutan Visa.