The answer is: probably you shouldn't!
If it's a large file, you should use Firebase Storage instead of the real-time database.
You can store large text files, CSV files, images, anything, up in the Storage, and it's easy to download them.
(However, if your app needs to allow users to change/modify small pieces of the file regularly, then it might make sense to store it in the Realtime Database in a structured way. In that case, you could do a big import of a JSON-formatted text file with the data inside of it.)