How to timestamp a PDF
There are two different things people mean by "timestamp a PDF", and they solve different problems. One embeds a signed time into the file itself; the other publishes a fingerprint of the file so its date can be proved independently. Here is how to tell which you need.
The two kinds of PDF timestamp
1. An embedded signature timestamp (RFC 3161 / PAdES). Acrobat and similar tools can attach a cryptographic timestamp from a Timestamp Authority to a digital signature inside the PDF. The time comes from a trusted third party, and it lives in the file.
2. A blockchain timestamp of the file's hash. You compute the SHA-256 fingerprint of the PDF and publish that fingerprint in a public ledger. The file is unchanged and never uploaded; the proof lives outside it.
They are not competitors so much as answers to different questions:
| Embedded timestamp | Blockchain timestamp | |
|---|---|---|
| Modifies the PDF | Yes — it is written into the file | No |
| Who you trust | The Timestamp Authority | A public ledger; nobody in particular |
| Still valid if the issuer shuts down | Depends on their certificate chain | Yes |
| Works on any file type | No, PDF only | Yes, any file |
| Needs the file uploaded | Usually yes | No |
| Best for | Formal e-signature workflows, eIDAS compliance | Proving a file existed and is unchanged |
If you need a legally-recognised electronic signature in the EU, use PAdES. If you need to prove this exact PDF existed on this date and has not been edited since, the blockchain route is simpler, cheaper and does not touch the file.
Timestamping a PDF with a blockchain record
- Hash the PDF. The SHA-256 fingerprint is computed in your browser — you can do it right now with the free hash generator. The file never leaves your device.
- Publish the hash. The fingerprint is written into an Ethereum transaction, which is permanent and public.
- Keep the certificate. You receive a file containing the hash, the transaction, and a verification link that anyone can check on Etherscan.
To verify later, re-hash the PDF and compare. If the hashes match, it is the same document; if they differ, it was edited.
The critical detail: don't touch the file
A hash covers every byte. Opening a PDF and re-saving it, flattening it, adding a comment, or letting a tool "optimise" it will change the hash and break the link to your timestamp.
So: timestamp the final PDF, archive that exact copy untouched, and work from duplicates. If you must revise it, timestamp the new version too — a dated series of versions is more useful than a single record anyway.
Embedded timestamps in Acrobat
For completeness, if you do need option 1: in Acrobat, configure a timestamp server under the signature preferences, then apply a digital signature. Adobe publishes free and paid TSA endpoints. The timestamp binds to the signature, so it proves when the signature was applied, not when the underlying content was authored.
What this proves, precisely
A blockchain timestamp on a PDF establishes that those exact bytes existed no later than that block, and that they have not changed since. It does not prove who wrote the PDF, and it does not prove the content is true — see proof of existence vs proof of authorship.
For contracts, drafts, reports and specifications, that is usually the fact in dispute: was this the version in existence on that date?