10. The system of claim 9 , wherein selecting the particular document version comprises: selecting the particular document version when a length of the particular document version exceeds a threshold length.
9. A system comprising one or more computers and one or more non-transitory computer-readable storage devices storing instructions that are operable, when executed by the one or more computers, to cause the one or more computers to perform operations comprising: identifying a plurality of different document versions of a particular document; identifying a first type of metadata that is associated with each document version of the plurality of different document versions, wherein the first type of metadata includes data that describes a source that provides each document version of the plurality of different document versions; identifying, by the computer system, a second type of metadata that is associated with each document version of the plurality of different document versions, wherein the second type of metadata describes a feature of each document version of the plurality of different document versions other than the source of the document version; for each document version of the plurality of different document versions, applying a priority rule to the first type of metadata and the second type of metadata, to generate a priority value; selecting a particular document version, of the plurality of different document versions, based on the priority values generated for each document version of the plurality of different document versions; and providing the particular document version for presentation.