This document is sometimes referred to as the charter. In the case of a bankruptcy, a receiver, trustee, or assignee must sign any return required to be filed on behalf of the corporation.You'll need to register your business with the state, prepare internal corporate documents, appoint company directors, and issue stock. Who must file Form IL-1120-ST? You must file Form IL-1120-ST if you are a small business corporation ("S corporation"), as defined in Internal Revenue Code. Learn how to setup an LLC or corporation in IL: affordable online incorporation services. Save time and money.