A Non Disclosure Agreement would still be valid if it is signed in a simple paper or in a stamp paper as it is considered to be valid agreement in any case. The stamp paper is used to ensure further validity and enforceability of the Non Disclosure Agreement. But it is not necessary for it to get printed in a stamp paper, the perks of doing it legally are much more than that of agreement in a normal paper. As a Non Disclosure Agreement is a agreement to protect the information, it would be advisible to make it as much secure as possible.
Have the Non Disclosure Agreement printed in judicial paper of minimum value and give the stamp duty, which will be applicable as per the State rules and sign the agreement before a witness, and then get it notorised. Doing all this will ensure that the valdity of the agreement will not be questioned in the court of law. Moreover the limitation period of the agreement with a witness is fro 12 years compared to a limitation period of the agreement without witness. The breach of such a agreement is misappropriation other than the damages mentioned in the clauses.
Reference: The Registeration Act 1908, Indian Contract Act 1872