In the United States, copyright exists for a term set by law. After that term expires, everyone may freely use the material—it will have entered the public domain. As of 2020, works published in the United States in 1924 and earlier are in the public domain. (The rules governing unpublished works are more complicated.) You do not need anyone’s permission to use them, but you may have to pay an archive or museum to reproduce them for you, and these institutions may limit the ways you can reuse their material.
Some documents are in the public domain from the start, such as those created by the U.S. federal government. State, county, and municipal documents are not necessarily in the public domain.
However, copyright law changed in important ways several times over the course of the twentieth century, so figuring out what is in the public domain and what remains protected by copyright can be confusing. See the guide to the public domain in the Resources section.