The most essential documents for Indian travel are passport and visa, along with their photocopies.
One of the two most important India Travel documents to carry with you is your Passport. Make sure to have this basic document with you all the time. Before the trip starts, check that your passport is in order and will be valid for the period of your stay. Otherwise, get it renewed. If, by any chance, your passport gets lost or stolen, immediately contact your country's embassy or consulate in Delhi. And don't forget to inform the nearest police station at the same time.
The second essential document for an Indian visit is a visa. Only in case of Nepal, you don't need any prior visa. The only requirement is of identification papers, and visa is issued at the airport itself. There are different types of visas for different travel purpose. Make sure to apply for the correct visa type, as per the purpose of your travel.
Don't forget to keep photocopies of your important documents for travel to India. Keep them separate from the originals, in case the originals get lost or stolen.
This article provides you information regarding essential documents for India travel.