Identifying a document by performing spectral analysis on the contents of the document

An article of manufacture comprising a non-transitory computer-readable medium having instructions stored thereon that, if the instructions are executed by a computer, cause the computer to perform functions comprising:

• creating an index, with one or more entries, for a rendered document, wherein each of the one or more index entries contains a word and one or more values, wherein each value represents a characteristic of the word;
• selecting a subset of the one or more index entries from the index;
• creating an ordered sequence of values from the selected subset of one or more index entries, wherein each value comprises a position of the entry relative to another entry in the selected subset of the one or more index entries;
• building an identifier for the rendered document based on the ordered sequence of the values, wherein each index entry in the subset satisfies an identifier rule for a document identifier;
• and
• locating an electronic counterpart to the rendered document based on the identifier for the rendered document.