Format: 

Author, A. A. (Year). Title of work (Report No. xxx). Retrieved from URL

Author, A. A. (Year). Title of work (Report No. xxx). Washington, DC: U.S. Government Printing Office.

Examples: 

U.S. Department of Commerce, Census Bureau. (2015, July 16). Who Votes? Congressional Elections and the American Electorate: 1978-2014 (Report No. P20-577). Retrieved from http://www.census.gov/content/dam/Census/library/publications/2015/demo/...

In-text: (U.S. Department of Commerce, 2015, p. 4)

Notes: 

Include the agency name in the retrieval statement if the agency that published the report is not already identified in the author field (i.e. Retrieved from U.S. Census Bureau website: URL).

Source:
APA - 7.03, no. 31