Final answer:
The Emancipation Proclamation was the document signed by Abraham Lincoln to free slaves in the United States.
Step-by-step explanation:
The document that Abraham Lincoln signed to free slaves in the United States was the Emancipation Proclamation. This document was signed in 1862 during the American Civil War and declared that all slaves in Confederate territory were to be set free. However, it did not immediately free all slaves in the United States, as it only applied to states in rebellion against the Union. It was an important step towards abolishing slavery and had a significant impact on the outcome of the war.
Learn more about The Emancipation Proclamation and the abolition of slavery in the United States