how to use bitcoin blockchain be useful to authenticate documents? have read a little about blockchains and found there is lot of blockchain npm libraries available.

How can I take benefit of the bitcoin blockchain to authenticate documents : to obtain a proof a document has not been alterated ot changed.

Any clues, suggestions, ideas and remarks are welcome.
You can make a transaction with the hash (of the document) as a comment. 
That would proof that the file existed at that time.

It doesn't however magically proof that a document hasn't been changed. 
Once you get the document, you can store the hash on the blockchain, then to check if it hasn't been changed, you can retrieve the hash and check if the current version of the file still hashes to the same value.

There will most likely be a library for this, I wouldn't know the name of a specific one however.