Handling Qualified Electronic Signatures (QES) in Apple Preview
What happens to a QES when I save a PDF in Apple's "Preview"?
As soon as a PDF file is saved or exported with the app Preview under macOS, a contained qualified electronic signature is usually invalid or completely removed.
Why is the signature destroyed by simply saving?
A QES is based on a mathematical fingerprint (hash value) of the document.
Structural change: Apple Preview often rewrites the internal object structure of the PDF file when saving to optimize it.
Loss of integrity: Even if the text content visually remains the same, the data bits in the back change. Since the signature guarantees the immutability of the file, any verification program detects a manipulation after saving and reports the signature as invalid.
Container loss: In many cases, Preview even completely deletes the cryptographic metadata of the signature from the file during the saving process.
Is the signature lost already when opening the file?
No. As long as you only view the document and do not save, the signature remains intact. The risk only arises when:
The command
File > Save(Cmd+S) is executed.The document is saved anew via
Export....The macOS "Auto Save" function is triggered by a small change (e.g., a highlight).
Can I repair an invalidated signature?
No. A once damaged or removed QES cannot be restored. The document must be reopened in its original state (before saving with Preview) or resigned by the issuer.
How do I properly handle signed PDFs?
To avoid jeopardizing the legal validity of a QES, the following rules should be observed:
| Recommendation | Procedure |
| View | Preferably use the Adobe Acrobat Reader, as it correctly recognizes and validates signatures. |
| Edit | Signatures are a "seal." If you need to change text, do so before the signature. Subsequent changes render the QES worthless. |
| Archive | Always save signed files by directly moving them in Finder (drag & drop), never by "opening and saving" in an app. |
How can I tell if the signature is still valid?
Do not use the Preview app for verification (it often does not show signatures or only as a static image). Instead, use:
The Adobe Acrobat Reader (lock icon/signature field).
Official verification portals (e.g., the Validator of the Swiss Federal Administration or the EU Commission portal)
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