1. A method of generating fine-grain fingerprints for identifying content in a rendered document, the method comprising: applying image-based techniques to identify patterns in a document rendered by an electronic document rendering system, irrespective of a file format in which the rendered document was electronically created, the applying of the image-based techniques including, i. identifying candidate keypoints at locations in a local image neighborhood of the document, by an electronic processor, and ii. combining the locations of the candidate keypoints to form fine-grain fingerprints indentifying patterns representing content in the document, by an electronic processor.