Format: 

Lastname, Firstname. Title of Report. Publisher, Day Month Year. Report number. Database Name or Title of Non-Publisher Website, DOI or URL. Accessed Day Month Year.

Government, Organizational Unit, Agency. Title of Report. Publisher, Day Month Year, URL URL or DOI (when accessed from the publisher's website). Report number. Accessed Day Month Year.

Lastname, Firstname. Title of Report. Publisher, Day Month Year.

Examples: 

File, Thom. Who Votes? Congressional Elections and the American Electorate: 1978-2014. United States, Department of Commerce, Census Bureau, July 2015, census.gov/content/dam/Census/library/publications/2015/demo/p20-577.pdf. Accessed 12 Dec. 2017.

In-text: (File 4)

Notes: 

If you do not know the individual author for a government document, use the name of the issuing government agency. You should always begin the citation with the government (e.g. United States or New York), followed by any organizational units, and then the agency (MLA 104). As with corporate documents, government reports are typically treated as standalone documents, similar to books. If the government agency listed in the author position is the same as the publisher, you can omit the author (MLA 24).

Source:
MLA - pp. 104-05