For any research project involving the activities of government, government documents are an indispensable resource. Examples of government documents would include the proceedings of legislative activities, official text of laws, regulations and treaties, statistical information, scientific data, photographic evidence, maps and more. What all of these varied materials have in common is that they were created by a governmental entity in the course of its daily business and provided to the public at goverrnment expense. Such documents are considered the "official voice" of the government.
Government documents are not all primary sources however, so, as always, be critical when assessing any given document for those characteristics that define primary sources.