Legal Question in Civil Litigation in California

You can subpoena witnesses and documents for trial. The procedure is similar, but not identical, to the procedure used in discovery. Note that you will probably need a witness to testify about the documents, since most must be authenticated before they can be admitted and since you will need someone to explain how they relate to your case. This often involves subpoenaing a witness from the company that holds the documents.

You can still subpoena a witness and documents for trial. You just might not have a chance to review them until the witness shows up with the documents for trial.

The judge is going to hate you for having a witness and documents produced at trial that you have never seen, and you are going to have a hell of a time making any use of them, but you can indeed use a subpoena duces tecum to compel production of documents at trial that have never been produced in discovery. And in fact you would need to use it to get originals produced even if you HAD seen them in discovery.