Tab Menu Web part which is based on cmstabcontrol.ascx (CMSWebParts/Navigation/cmstabcontrol.ascx) displays Document names by default. This means you can add a new property to your document type for your tab name and customize the Tab Menu Web part to display it instead od Document name. Read the documentation of CMSTabControl https://docs.kentico.com/display/K8/CMSTabControl and BasicTabControl which is inherited by CMSTabControl https://docs.kentico.com/display/K8/BasicTabControl