1. A computer-implemented method for searching unstructured documents for structured data, at least a portion of the method being performed by a computing device comprising at least one processor, the method comprising: receiving a request to search unstructured documents for a document that contains: a value from a first field of a dataset; and a value from a second field of the dataset; generating a secure search index for searching the unstructured documents by, for each record in the dataset: identifying, within the dataset, the record's value from the first field and the record's value from the second field; generating a first hashed key from the record's value from the first field and the record's value from the second field; adding the first hashed key to the secure search index; extracting a first token and a second token from an unstructured document; generating a second hashed key from the first token and the second token; querying the secure search index to determine whether the second hashed key is contained in the secure search index; responding, upon determining that the second hashed key is contained in the secure search index, to the request with information about the unstructured document.