Anything that is NOT protected under copyright law, this includes ALL works published before 1923 AND under certain conditions, works published up to 1978.
A ‘work’ can be anything; a book, a play, music, photographs, movies, instruction manuals poster, etc…….
Re-published and re-packaging Public Domain information & other creative works is not illegal. The reason is fairly simple. If you acquire, re-package and sell information that has fallen into the public domain you do not have to pay royalties or copyright fees on that work.