A government document can be anything produced by an agency acting as an agent of a city, regional, state or national entity. These can come is a variety of formats:
Most US government agencies will use the .gov domain in their URL. Adding to the mix (and sometimes confusion) are quasi-government agencies such as the US Postal Service, the Smithsonian Institution, and the Federal Reserve which do not use the gov domain name but are tightly tied to the Federal government by function or funding. The US military has its own URL domain (.mil).
As important as government information can be to a historian, not everything has been digitized and in some cases may never be due to national security issues, presidential privilege or legislation. A lot of information has been declassified over the years through normal records management or Freedom of Information Act (FOIA) requests but not be hosted by the originating agency, making discovery a challenge.
More information on government agencies and resources is in the Government Documents Research Guide. I am here to help you so please contact me for assistance.
The material on the internet can be altered or even removed (such as during a change in administration.)
There are some tricks you can use to narrow your search results to specifically government-hosted primary collections.
Catalog of US Government Publications (AKA the Monthly Catalog) will list all publications and provide links where possible. If the item hasn't been digitized, you can request it using your institution's Interlibrary Loan service. USA.gov is for current Federal services and resources.
Search a specific agency:
Search a specific state, city or other subdivision, generally they will have the .gov domain in their URL.
Search the Library of Congress at https://www.loc.gov/
CSUSM users have access to the US Congressional Serial Set and the American State Papers in the Databases by name collection.