Glossary
What is a tamper-evident seal?
Short answer
A tamper-evident seal is a digital signature that an e-signature service applies to the finished PDF, covering every page and the evidence certificate. It does not stop anyone editing the file, but any edit breaks the seal and Adobe Acrobat and other readers show a warning. Sign PDF seals every completed document this way, with an independent DigiCert timestamp.
Updated 25 October 2026
- Tamper-evident seal
- A tamper-evident seal on a PDF is a digital signature applied over the completed document so that any later change to the file invalidates the signature and is flagged by PDF readers.
How a tamper-evident seal works
The service computes a hash of the whole PDF and signs it with its private key, embedding the signature and certificate in the file. A PDF reader recomputes the hash on opening and checks it against the signature.
If the hashes match, the reader reports the document has not been modified since it was signed. If someone changed a date, deleted a clause or swapped a page, the hashes differ and the reader reports the signature as invalid or the document as altered.
Tamper-evident, not tamper-proof
The word matters. Anyone with the file can still change it, just as anyone can tear a paper envelope. The seal makes the change visible. That is what you need in a dispute: the honest party holds a copy whose seal is intact, and the altered copy shows its alteration.
Seal versus signer signatures
A seal is made with the platform's key, not the signers'. It proves the file has not changed since the platform sealed it, and the timestamp proves when that was. It does not prove who the signers were; that comes from the audit trail.
eIDAS has a separate concept called an electronic seal, defined in Article 3(25), for legal persons such as companies. Sign PDF's seal is a digital signature for integrity and is not offered as a qualified electronic seal.
How the seal works in Sign PDF
When the last signer finishes, we write the fields into the PDF, add the certificate of completion, and seal the whole file with a PAdES-style signature carrying a DigiCert timestamp. Open it in Acrobat Reader and the signature panel shows it as valid and unmodified.
Edit the file afterwards and the seal breaks. For a second check that does not depend on any PDF reader, drop the file into signpdf.ai/verify to compare its fingerprint with the original.
Questions people ask
Can I add a page to a sealed PDF?
You can, but the seal will show the document was changed after sealing. If you need changes, send a new version for signature.
Why does my PDF reader say the seal is valid but the identity is unknown?
The integrity check passed, but the signing certificate does not chain to a root the reader trusts on that computer. The document is still unchanged.
Does printing a sealed PDF keep the seal?
No. The seal is digital and exists only in the file. A printout is a copy; keep the electronic original as your evidence.
Is a tamper-evident seal the same as encryption?
No. Anyone can read a sealed PDF. The seal protects integrity, not confidentiality.
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