Check a PDF for changes
Whether a PDF was changed after signing can only be proven in two ways: by a signature inside the file, or by comparing it with a version stored somewhere else. Anyone who receives a canusign document can check it both ways, without an account, and the file never leaves the device.
What metadata shows, and what it does not
A PDF carries details such as the creation date, the modification date and the program that produced it. You find them in the document properties of your PDF reader. They are clues, not proof: whoever edits the file can change or remove them with little effort.
What counts as proof
- A signature inside the file: Adobe Acrobat Reader shows the status of each signature in its signatures panel, and Microsoft Edge can also view and validate certificate signatures in PDFs. A signature pasted in as an image is not such a signature.
- A comparison with a stored version: if the hash of the file matches the hash of a version held by an independent party, it is the same file, byte for byte.
By document ID
Every signed PDF carries a document ID on each page and in the audit certificate. Enter it on the verification page or scan the QR code in the certificate. The page shows who signed when, and the hash of the stored version.
By file
On the result page, drop the PDF onto the upload area. The browser computes the hash itself and compares it with the version stored at canusign. The file is not uploaded.
In Adobe Acrobat
The timestamp comes from DigiCert under RFC 3161 and is embedded with DSS and LTV. Acrobat shows it in the signature properties and lists it as trusted.
PDFs that do not come from canusign
The verification page only knows documents signed with canusign. It cannot check an arbitrary PDF. If the PDF carries a signature, check it in Acrobat Reader. If you stored a copy yourself, compare the hashes of both files: on a Mac in Terminal with shasum -a 256 file.pdf, on Windows in PowerShell with Get-FileHash file.pdf. Without a signature or a stored version, a change cannot be proven.
What a check does not show
It proves that the file is unchanged since signing and when it was signed. It does not prove the identity of the signers: canusign issues a simple electronic signature under eIDAS, valid where the law does not require a handwritten signature or notarisation.