An electronic publishing platform maps supplemental content layers associated with one or more paired target documents, such as user-generated content layers associated with the paired target documents, into a source document. The publishing platform correlates content items in the source document to content items in the paired target documents. Based on the correlation, the publishing platform generates indexes mapping the source document content items to the paired target document content items. The publishing platform uses the indexes to map supplemental content layers associated with the paired target document to the source document.