1. A computer implemented method of selecting supplemental content for a document, the method comprising: receiving, by a data processing system, a notification of a request coming from a user device for a document; parsing the requested document into one or more tokens and determining a token priority score for each token; selecting a subset of tokens from the one or more tokens, each token in the subset of tokens having a token priority score above a predetermined threshold value; accessing, by the data processing system, a set of Boolean query expressions stored in association with a plurality of supplemental content records; identifying a subset of Boolean query expressions from the set of Boolean query expressions, each Boolean query expression in the identified subset of Boolean query expressions satisfied by the selected subset of tokens; determining a document relevance score for each Boolean query expression in the identified subset of Boolean query expressions, wherein each determined document relevance score reflects a relevance of the requested document to a corresponding Boolean query expression; selecting a Boolean query expression from the subset of Boolean query expressions that is best satisfied by the requested document based on the determined document relevance scores; selecting a supplemental content record associated with the selected Boolean query expression; and outputting data for supplementing the requested document with content indicated by the selected supplemental content record.