How can you tell if a signed PDF was modified?

An altered PDF doesn't look altered. Someone opens a signed contract, changes an amount, saves, and the resulting file looks exactly the same: same signatures, same layout, same certificate page at the end. There is no way to spot it by eye.

That is the problem verifiable integrity solves, and it is a different problem from legal validity. Proving who signed is one thing; proving that what was signed is still the same is another.

Firmiu doesn't rely on a single mechanism. Every completed document ships with three independent safeguards, and any one of them is enough to expose tampering.

The first is a cryptographic signature embedded inside the PDF, in PAdES format. It isn't a drawn stamp: it's a mathematical computation over the file's exact bytes. Change a single character and the signature no longer matches, and PDF readers that validate signatures report it on their own when the document is opened.

The second is the SHA-256 fingerprint, printed on the certificate page. It's a 64-character identifier computed from the content. Two identical files produce the same fingerprint; two files differing by one comma produce completely different ones.

The third is the reference number: a unique identifier, also printed on the certificate, recorded on Firmiu's servers along with the document's fingerprint and each signer's details.

The fastest route requires nothing: go to firmiu.com/verificar and enter the reference number shown on the PDF's certificate page. You'll see the document name, who signed, when, and the recorded fingerprint. No Firmiu account is needed, which is precisely why a bank, a client or a judge can do it.

If you also have the file at hand, upload it on that same page. Your browser computes the PDF's fingerprint and compares it with the one recorded at signing time. A match means it's exactly the document that was signed. No match means it was modified, or it's a different file.

One detail matters here: that computation happens inside your browser. The PDF is never uploaded to Firmiu or to any server. Only the fingerprint travels — 64 characters that cannot be used to reconstruct the content.

The third route is opening the PDF in a reader that validates digital signatures, such as Adobe Acrobat Reader. There the program itself checks the cryptographic signature and tells you whether the document was modified since it was signed. Note that Chrome's and Edge's built-in viewers do not validate signatures, so you won't see any warning in them.

Integrity answers whether the document changed. The audit trail answers who signed and under what circumstances, and Firmiu generates it automatically without anyone having to switch it on.

For every signer it records the name and the email the link was sent to, the date and time in their own timezone, the IP address they signed from, the browser and operating system, the approximate geographic location, and whether a VPN or proxy was detected. Each signer also confirms with a four-digit verification code delivered only to their inbox.

All of it is printed on the certificate page appended to the end of the PDF, alongside a thumbnail of each signature. It isn't a separate annex that can go missing: it travels inside the same file.

When an agreement is disputed, the question is rarely whether it was signed. It's usually whether what appears on the page today is what was signed back then.

Without an integrity mechanism, that discussion becomes one person's word against another's. With the fingerprint on record and the cryptographic signature inside the file, it stops being an opinion: either the document presented matches the record or it doesn't, and that takes seconds to check.

It's worth being precise about what this means. The cryptographic signature certifies that the PDF left Firmiu's signing flow with that exact content on that date. Each signer's identity is backed by the audit trail: the email the link reached, the verification code, the IP and the device. The two pieces of evidence complement each other.

A PDF signed with Firmiu carries a cryptographic signature inside it, a printed SHA-256 fingerprint and a reference number that can be looked up online. Any of the three exposes a later modification, and checking costs nothing and requires no account.

It's available on every plan, including the free one, because it's a property of the document rather than a feature worth charging for separately.