12. A non-transitory machine-readable medium storing a program for generating a low-resolution version of an electronic document at a content distribution system, the program executable by at least one processing unit, the program comprising sets of instructions for: from a content publishing application, receiving (i) a high-resolution version of the electronic document that comprises a plurality of high-resolution images and (ii) a markup language file associated with the high-resolution version of the electronic document, the markup language file comprising a plurality of annotations that each specify a resolution for a low-resolution version of a respective high-resolution image, the specified resolution identifying a maximum pixel width and pixel height resolution at which the respective image will be displayed on client devices having a particular resolution, wherein the low-resolution version of the electronic document is not received by the content distribution system from the content publishing application; creating, at the content distribution system, the low-resolution version of each respective high-resolution image of the plurality of high-resolution images based on the respective annotation of the plurality of annotations; generating, at the content distribution system, the low-resolution version of the electronic document using the created low-resolution image; and storing (i) the high-resolution version of the electronic document with the high-resolution image for distribution to a first plurality of client devices and; (ii) the low-resolution version of the electronic document with the low-resolution image for distribution to a second plurality of client devices having the particular resolution.
16. The non-transitory machine readable medium of claim 12 , wherein the program further comprises sets of instructions for: at the content distribution system, receiving (i) a first request from a first client device to access the electronic document and (ii) display capabilities of the first client device that indicate a resolution of the first client device; sending the low-resolution version of the document from the content distribution system to the first client device based on the display capabilities of the first client device; at the content distribution system, receiving (i) a second request from a second client device to access the electronic document and (ii) display capabilities of the second client device that indicate a resolution of the second client device; and sending the high-resolution version of the document from the content distribution system to the second client device based on the display capabilities of the second client device.