Short answer: add the old file and the new file, press Compare. You get the added and removed lines per page. Identical text reports no change; two scans report that there is nothing to compare instead of claiming the files match.
What “no differences” really means here
It means the extracted words are identical. A page that swapped a logo, moved a signature box or changed a font reports no difference, because none of those live in the text layer. Read the result as “the wording did not change”, never as “the documents are the same”.
Why scans need OCR first
Extracting text from a photograph of a page returns nothing, so a naive comparison would call two different scans identical. This tool checks both sides for a text layer and stops when neither has one — the honest answer is that it cannot compare pictures.