I'm not an expert. But I do know it's a minefield. Public domain is a legal status, and is not the same as being publicly accessible. Finding something on the internet is no guarantee of any kind. If you want to reproduce any work by anyone with the intention of enhancing your own work, it's worth your trouble to track down the original publisher and/or copyright owner, and getting formal approval. If you don't, you could end up paying through the nose.