If a work is in the public domain, that means it isn't copyrighted. There are three ways for a work to enter the public domain:
Currently, the copyright lasts for the life of the author plus 70 years, and then the work goes into the public domain.
However, U.S. copyright law has undergone a lot of changes over the years and copyright expiration dates can be complex. To check whether the copyright on a specific work has expired, consult a public domain chart or timeline: