A "Pelican Brief" is a legal thriller novel written by John Grisham and published in 1992. The novel involves a law student who stumbles upon a conspiracy involving the assassination of two Supreme Court justices and must work to uncover the truth and bring those responsible to justice.
In general usage:
The term "Pelican Brief" has sometimes been used more broadly to refer to any legal brief or document that is particularly complex or controversial, or that involves allegations of conspiracy or misconduct.