Which Indian scripture is equal to the Koran or Bible?
Actually there is none. It is very difficult to say.
The Vedas are considered as the main source (of knowledge), but it is so vast. For all practical purposes, Bhagavad Gita is what everyone reads because it has the essence of most of the main knowledge. So Upanishads and Bhagavad Gita can be considered as the Bible or Koran of the East. That is what is used in the courts also.
The Gita is only 5,200 years old, but this tradition has been there before the Gita, from several millenniums. In Rama’s time, there was no Bhagavad Gita, only the Vedas were there, but the Vedas were in such an ancient language which people could not understand. The Vedas need a lot of interpretation, and they are very symbolic in many places.
The Upanishads and Vedas are considered the authority, and the Bhagavad Gita for practical purposes is the Koran or Bible of the east.