1. A computer-implemented method comprising: comparing image content from a first document to image content from a reference document, wherein the reference document is associated with at least one keyword; determining, by one or more processors, a similarity measure between the first document and the reference document based on the comparison of the image content; associating one or more of the keywords from the reference document with the first document based on the similarity measure between the first document and the reference document; receiving a request for a document, wherein the request includes at least one of the one or more keywords; and responsive to the request, delivering the first document.