In my fiction, the Queen keeps her own archives in the Palace Library but there is a restricted section where you need authority to read on so most do not bother. In any setting there must be some information where restrictions of access are necessary, almost certainly on security grounds, so what or who is the threat here? Who is your historical expert in your fiction? Is there anyone writing alternative histories (because they disagree with the official version or feel things have been left out)?
What controversial documents are in your fictional archives? Could the archive documents unleash choas if they were made public or is it a case everyone knows what is is them, it is just the fine details that cannot be published?