They store every word that MAY be in the scanned document.

So their OCR engine will find a lot of legitimate words, but it will also find a lot of words that don't sense too.

When putting in a term for searching, it looks at the entire index (both legit words and the garbage) and returns you the documents that match.

I think it's quite clever.

Bear in mind that this feature was many years ago, I have no idea if this is still the case.