1. A method comprising: accessing, at one or more computing devices, a document; identifying a plurality of sentences of the document, each sentence identified based on punctuation in the document; and executing, for each of the sentences of the document and using the one or more computing devices, a document modification operation that includes: generating a ranking score for each of a plurality of passages from external documents, wherein the ranking score is based at least on a degree of semantic similarity of each of the plurality of passages from the external documents with respect to the sentence of the document, modifying the sentence to include a footnote link for the sentence in the document, the footnote link including a link to the external document having a highest ranked passage therein if the ranking score of the highest ranked passage with respect to the sentence exceeds a threshold value, and skipping modification of the sentence if the ranking score of the highest ranked passage with respect to the sentence does not exceed the threshold value.