16. A non-transitory computer-readable medium having stored therein instructions, that when executed by one or more computing devices, cause the one or more computing devices to perform functions comprising: receiving first text captured by a first mobile device from a first instance of a rendered document; identifying, from a repository of digital documents, a digital form of the rendered document that corresponds to the first captured text; receiving, from the first mobile device, supplemental content for the digital form of the rendered document; storing the supplemental content, in a database of supplemental content associated with the digital documents; receiving, from a second mobile device, second text captured by the second mobile device from a second instance of the rendered document; identifying, from the repository of digital documents, the digital form of the rendered document that corresponds to the second captured text; determining that the supplemental content exists for the digital form of the rendered document; adding one or more indicators to the digital form of the rendered document that indicate that the supplemental content for the digital form of the rendered document exists; and providing the digital form of the rendered document along with the one or more indicators of supplemental content to the second mobile device.
19. The non-transitory computer-readable medium of claim 16 , wherein the supplemental content includes a purchase opportunity associated with the rendered document.