Congressional documents originate from congressional committees and cover a wide variety of topics and may include reports of executive departments and independent organizations, reports of special investigations made for Congress, and annual reports of non-governmental organizations. Congressional documents, along with Congressional reports, are part of the U.S. Congressional Serial Set, commonly referred to as the Serial Set.
(Source: Govinfo)
For more information on the various types of congressional documents, visit this site.
There are a number of online resources where you can retrieve congressional documents. Some are available to the public, and some the University of Chicago subscribes to. Begin with the following: