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A work in the public domain is not protected by copyright. You are free to copy the entire work. In the United States, works published before 1923 are in the public domain.
The majority of documents published by the federal government are in the public domain. There are some exceptions. The federal government outsources some of its research and publications to private publishers. Those works may be copyrighted. Check the specific document. For information on how to find and cite these documents, please see the Federal Government Documents Resources Guide
Also many state and county publications may be copyrighted; they are not necessarily in the public domain.
This is a difficult question with no quick answer. The copyright for some works published prior to the 1976 law may not have been renewed. Lolly Gasaway's chart shows the changes in copyright terms from 1923 forward. The Copyright Office maintains a database of copyright registrations back to 1978. The Copyright Office also publishes a circular How to Investigate the Copyright Status of a Work. Prior to 1978, the Copyright Office can do a manual search of it records for a fee.
A work can also be an "orphan work" meaning that it may still be under copyright, yet no rightsholder can be found.
For those who wish to explore further...