Will a signed document still verify in 20 years?

What "still verifies" means after 20 years

A signed document verifies when someone can show, from the file alone or almost alone, who signed, what they signed and when. For that to work in 20 years, four things must still hold:

1. The file opens. Its format is still readable.
2. The checks still run. The software that checks it, or the instructions to rebuild that software, still exist.
3. The cryptography still holds. The algorithms behind each signature and fingerprint haven't been broken, or the record proves the signature existed before they were.
4. The outside facts can still be read. Anything the check consults beyond the file, such as a certificate list or a timestamp, is still available.

Why many signed PDFs need renewal

Many e-signature services seal the finished PDF with a certificate-based signature: a certificate authority vouches for a signing key for a limited period, so checking it years later needs proof that the certificate was valid at signing. The standard long-term formats embed that validation material and add periodic timestamps, because, as the European Commission's Digital Signature Service documentation puts it, "The validity could be limited due to cryptographic obsolescence of the algorithms, keys and parameters used, or due to expiration or revocation of the validation material."

EU law treats this as its own service. Under eIDAS, a qualified preservation service must use "procedures and technologies capable of extending the trustworthiness of the qualified electronic signature beyond the technological validity period" (Article 34(1)).

Vendors describe the same limit in their own words, as published on October 11, 2026:

• Adobe says a regulated digital signature "can be validated for up to 10 years", and that "both the document and the signature are designed to be revalidated for at least 10 years".
• Signaturit, in a help article of April 12, 2021, says downloaded documents' "validity won't last forever, as certificates must be updated".
• Clicksign (Brazil): to validate a document signed more than 12 months earlier, Clicksign's certificate must first be renewed through Clicksign's own validator.

Long-term validation is the sound answer for certificate-based signatures, and it means care over the years: renewed timestamps, or a service that renews them.

What lasts in an Attest signed copy

An Attest signed copy has no certificate that can expire. Each signature is checked against the signer's passkey public key, stored in the evidence, and its time against a public blockchain. (A passkey is a cryptographic key only the signer controls, unlocked with Face ID, Touch ID, Windows Hello or the device PIN.) Every party downloads the same sealed PDF: the signed pages, with a "Signed with Attest" stamp on each signature and a seal line at the foot of every page, a certificate of completion, the audit trail up to completion and a "Verify it yourself, without us" page, with the full evidence, the original document and a standalone verifier attached inside.

What	What it is built on	Why it lasts
The signed PDF and the original inside it	PDF, an ISO standard (ISO 32000)	The Library of Congress describes PDF as "Extremely widely adopted"
The evidence (attest-evidence.json)	Plain-text JSON	It lists every check in plain words, so each one can be repeated with standard tools
Fingerprints	SHA-256, a NIST standard (FIPS 180-4)	A fingerprint is a code calculated from a file's bytes; any SHA-256 tool recomputes it
Each signature	WebAuthn, a W3C standard	Any WebAuthn library can re-check it against the passkey public key in the evidence
The time	An anchor on Horizen, a public blockchain (a public ledger anyone can read)	Attest's registry contract has no function that changes or removes an anchor: once a wallet anchors a value, its time is fixed
The checker (attest-verifier.html)	One HTML file, open source under the MIT license	It carries its libraries and fonts inside, its build is reproducible, and its format specification lets anyone write their own

Only fingerprints go on the blockchain, never the document, its title or anyone's email. Drop the signed PDF on the verifier and it checks each signer's passkey signature, the original document, the pages you see and Attest's anchored record on Horizen. A signed copy keeps the verifier that was current when it was made, and each new release of the verifier reads every earlier evidence format.

What it still depends on

Four things are worth stating plainly: the file itself, and three things outside it.

1. The file itself. Every check starts from the signed copy: lose every copy and there is nothing to check, and a copy that was re-saved or converted no longer matches what was anchored. Keep the file exactly as downloaded, in two places (below).
2. A Horizen node, for the on-chain checks. The verifier reads a public node of Horizen (chain 26514) for its on-chain checks: Attest's anchored records of the agreement and of its pages, and that the verifier attached to the copy is a build Attest published. It accepts any node of that chain, because it checks the chain id, and every other check runs offline. If no node of that chain could be reached, those on-chain checks couldn't run.
3. Something that runs the checks. The verifier is a web page that runs in a current browser. If a future browser couldn't run it, the evidence file still lists every check, the verifier's public format specification (FORMAT.md on GitHub) describes each one so it can be rebuilt in any language, and each cryptographic check can be run with standard tools: any SHA-256 tool, any WebAuthn library and any Ethereum JSON-RPC node. Re-making the pages uses Attest's open-source renderer, published with the verifier.
4. Algorithms that hold. An IETF standard for long-term evidence records puts it this way: "During the archival period, hash algorithms and public key algorithms can become weak or certificates can become invalid", so "it is necessary to prove that the digitally signed data already existed before such a critical event" (RFC 4998). NIST's draft plan for the move to post-quantum cryptography (November 2024) lists ECDSA, EdDSA and RSA signatures as quantum-vulnerable and would disallow them after 2035, and it says its hash function standards, SHA-256 among them, are "significantly less vulnerable to known quantum attacks" than its public-key standards.

That last point is where the anchor earns its place. Attest's anchor of each agreement commits to every signer's statement, passkey public key and signature, at the time of its block. A signature swapped in years later would not match what was anchored, and any new anchor would carry its own, later time. What ties the anchor to the agreement is SHA-256. If SHA-256 were ever weakened, the established answer is the one RFC 4998 describes, and it leaves the work to whoever keeps the records, a long-term archive service or the owner of the data: before the old algorithm fails, they fingerprint the records again with a newer one and have the new fingerprints timestamped. For your signed copy, that is you, or the archive you keep it in.

How to keep a signed document checkable for 20 years

1. Download the signed copy when the agreement completes. Wait until it reads "every party signed, sealed and anchored"; a copy made while anchoring is still in progress is marked preliminary.
2. Keep the file exactly as downloaded. Re-saving, printing to PDF or converting it makes a different file with a different fingerprint, even if every word looks the same.
3. Store it in two places you control, such as your own drive and a separate backup.
4. Verify it once now with the verifier inside it, or on attest.datahubz.com/verify, which fingerprints it in your browser and confirms it is the exact file that was recorded and anchored, so you know it checks out before you need it.
5. Keep it as long as the law asks. Retention periods depend on the country and the document. Mexico, for example, requires merchants to keep contracts for at least ten years (Código de Comercio, article 49), with conservation certificates from accredited providers (NOM-151); where a law prescribes a method like that, keep the signed copy under that method too.

This page is general information, not legal advice.