A clean result means nothing was found in the extracted text, not that the documents match This compares the text layer. Scanned pages, drawing geometry, form values, annotations and attachments are not compared at all, and a page with no text layer compares as empty. Silence here is not evidence that two revisions are the same. Open for the full scope limits.
What this tool actually compares
The text extracted from each PDF, aligned and diffed at word or character granularity, with an optional pixel comparison of the rendered pages. Everything is local, and the diff engine itself is tested.
What it compares is the text layer. That is not the same as the document.
Not compared, at all
- Anything without a text layer. Scans and image-only PDFs extract nothing, so two entirely different scanned pages can both compare as empty and report no differences. There is no OCR here.
- Vector drawing content. Lines, dimensions, hatching, symbols and geometry on an engineering drawing carry the change, and none of it is text. A revised drawing frequently differs only in geometry.
- Annotations, comments, stamps and markups, which is where review changes usually live.
- Form field values, including filled AcroForm and XFA data.
- Embedded attachments, layers and hidden content such as optional content groups turned off in the view.
- Digital signatures and certification. Nothing here tells you whether a document is signed, whether the signature is valid, or whether it was altered after signing.
- Redaction quality. A black rectangle over live text is not a redaction, and this tool does not flag one.
Text extraction is also imperfect where it works: reading order, ligatures, hyphenation, columns, tables, superscripts and embedded subset fonts can all shift or drop characters, which shows up as a difference that is not there or hides one that is.
Never rely on this alone for
- Contract, legal or regulatory review, or confirming an executed version.
- Engineering drawing revision control, or approving an issued-for-construction change.
- Specification, standard or procedure revision control in a quality system.
- Confirming a document has not been tampered with, which is a signature and hash question, not a diff question.
- Clinical, safety or financial documents where a missed change carries consequences.
The dangerous failure mode of a diff tool is not a wrong difference, which you will notice. It is a missed one, which looks exactly like agreement.
Before you conclude two documents match
Check the revision block, the change log and the issuing authority. Compare hashes if you need to know the files are identical. Read the pixel comparison as well as the text diff, and put a human on anything that matters. For drawings, compare the revision clouds and the drawing register, not the text.