1. A document reading device comprising: a document reading section configured to optically read an image of an original document; an image memory configured to temporarily store the image read by the document reading section; and an image processing section configured to retrieve the image of the original document from the image memory and process the image, wherein the image processing section comprises: a mark recognizing section configured to recognize an image of a predetermined mark made on the original document; a sticky note recognizing section configured to recognize an image of a sticky note placed on a predetermined position of the original document relative to the mark; a mask determining section configured to determine in which mask pattern the mark is masked with the sticky note; and an image manipulating section configured to manipulate the image of the sticky note according to the mask pattern determined by the mask determining section.