The forged Declaration of Independence is a copy of the Declaration of Independence. It can be made into a replacement Declaration by the Lone Wanderer for the Rivet City historian Abraham Washington.
Once you give the ink container, retrieved from the Arlington Library's media archive to Button Gwinnett, he will create a forged Declaration. This will make it possible to leave the original Declaration in place, while giving Abraham Washington the forged one, making him believe it's the original.
Though it no longer serves any purpose, this item is still found in the game files for Fallout: New Vegas.