Public domain means ownership by the public, so to the extent you trust our (or your own, which is highly suggested, because it's easy and can save a lot of trouble in the future) copyright research, treat them like you own them.
That assumes you're based out of the United States, where copyright is of a (mostly) fixed-year monopoly that's all or nothing. Other countries don't believe in a public domain at all, protect "moral rights" beyond copyright (the right to be credited, for example), have their own laws that override the source country, or have no interest in United States copyright at all.