Once the term of the Non-Disclosure Confidentiality Agreement ends, it is mandatory to deliver all documents, materials, products and alike to the Disclosing party.
As soon as the term of the non-disclosure confidentiality agreement ends, the Receiving party must deliver all documents, materials, products and alike to the Disclosing party, even since the non-disclosure confidentiality agreement itself normally says so.
It's normal to say something like, "ending the business relationship that led one of the parties to disclose the secret to the other party, all information shared under this non-disclosure confidentiality agreement, regardless of the format (e.g., computer file, paper, product, etc..), must return to the sphere of the Disclosing party".
This is pretty obvious since if, in order to perform a given task, the Receiving party needed elements or documents from the Disclosing party, once the task is completed, the former should not keep anything that is not theirs. The same is not true if the non-disclosure confidentiality agreement provides for otherwise, which is an exception.
To learn more about non-disclosure agreements, refer to more frequently asked questions about non-disclosure agreements.