Am I allowed to use archived or public domain works that are found online?
Works that lack copyright protection, and thus fall in the public domain, are available for everyone to use freely and without permission. A work may “fall into” the public domain for a variety of reasons but most commonly for any of the following three
it is a government publication;
the copyright term has expired; or
the copyright owner has freely committed the works to the public domain.
You should not assume that just because a work appears online it is free to use. The Copyright Office provides a searchable database that can be used to check the copyright status of registered works. The Copyright Office also publishes a circular that discusses investigating the copyright status of a work is here. Unfortunately, copyright status searches are not conclusive in all cases. There are various situations in which a work may be protected even if no information is available about the work in the Copyright Office records.