A document annotation system includes a graphical user interface used by an annotator to annotate documents. An active learning component trains an annotation model and proposes annotations to documents based on the annotation model. A request handler conveys annotation requests from the graphical user interface to the active learning component, conveys proposed annotations from the active learning component to the graphical user interface, and selectably conveys evaluation requests from the graphical user interface to a domain expert. During annotation, at least some low probability proposed annotations are presented to the annotator by the graphical user interface. The presented low probability proposed annotations enhance training of the annotation model by the active learning component.