Ask your attorney. Documents obtained by subpoena are not necessarily admissible at trial and must still comply with the rules of evidence. The answer as to admissibility at trial depends on the type of documents you want to have admitted. evidentiary issues need to be spotted and taken care of well in advance of trial. Here's more: BLUE LINK BELOW
Law Offices of Andrew D. Myers, North Andover, MA & Derry, NH provide answers for informational purposes only. Actual legal advice can only be given by an attorney licensed in your jurisdiction, thoroughly familiar with the area of the law in which your concern lies. This creates no attorney-client relationship.
If your attorney intends to authenticate subpoenaed documents at trial only by a certificate of the custodian of business records then YES, the envelope needs to remain sealed until trial. There are other good reasons for the envelope to remain sealed until trial, depending on the circumstances. Your attorney can explain them to you.
Follow me on Twitter @joeroselaw. I answer questions on Avvo to try to help get you pointed in the right direction. But, I am not your attorney. Beware, my answers here are general, limited, incomplete, and can never be as complete, thorough, or accurate as one I would give to a client after hearing all of the facts and details of my client's situation and applying the correct law. Also, I am admitted to practice law only in California and all of my answers are intended exclusively for the Golden State.