15. An apparatus, comprising: one or more processors; and one or more memory devices for storing program instructions used by the one or more processors, wherein the program instructions, when executed by the one or more processors, cause the one or more processors to: access, at one or more computing devices, a document, identify a plurality of sentences in the document, each sentence identified based on punctuation in the document; and execute, for each of the sentences of the document and using the one or more computing devices, a document modification operation that causes the one or more processors to: generate 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, modify 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 skip modification of the sentence if the ranking score of the highest ranked passage with respect to the sentence does not exceed the threshold value.
15. An apparatus, comprising: one or more processors; and one or more memory devices for storing program instructions used by the one or more processors, wherein the program instructions, when executed by the one or more processors, cause the one or more processors to: access, at one or more computing devices, a document, identify a plurality of sentences in the document, each sentence identified based on punctuation in the document; and execute, for each of the sentences of the document and using the one or more computing devices, a document modification operation that causes the one or more processors to: generate 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, modify 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 skip modification of the sentence if the ranking score of the highest ranked passage with respect to the sentence does not exceed the threshold value. 16. The apparatus of claim 15 wherein the degree of semantic similarity of each of the plurality of passages from the external documents with respect to the sentence of the document is based on the entirety of the sentence.