The PDF clearly shows a signature - yet the reader flashes a warning: validity unknown, signature not verified. It feels like an accusation. Usually it is something much simpler, and sometimes it means there was never a certificate signature there at all. Here is how to read the message correctly.
Signature warnings confuse everyone because three different situations share one alarming vocabulary. One: the file was signed with a signature image (the free route), so there is no certificate for the reader to verify - nothing is wrong, there is simply nothing cryptographic to check. Two: the file holds a genuine DSC signature, but your reader does not yet trust the issuing authority, so it reports the signer as unknown until trust is set up. Three: the document was changed after signing, which breaks a real signature - the one case that deserves alarm. This page teaches you to tell them apart in a minute, and what to do in each case.
If the PDF was signed by the free image route, opening it shows your signature on the page - and typically no signature entry in the reader's signature panel at all. Some viewers still display a generic note about signatures because the file was annotated. This is not an error and not a broken signature: the file simply contains a picture, and a picture cannot be verified, only seen.
If you are the receiver and the document is an everyday one, judge it as you would a scanned signed page - by context and sender. If the document was supposed to carry a DSC, this is the moment to say so: the sender used the wrong route.
A genuinely DSC-signed document can still open with "validity unknown" or "signer's identity unknown" on a fresh computer. The mathematics checked out; what is missing is trust - your reader keeps a list of certificate authorities it believes, and the signer's CA may not be in your list yet, or the reader has not fetched the chain.
The fixes are reader-side and legitimate: open the signature properties and view the certificate chain, update the reader's trusted certificates (desktop readers offer an update for their trust lists), and on organisational machines follow your IT policy for adding the national root. Once trust is established, the same file shows a clean valid result - the document never changed, only your reader's knowledge did.
"Signature invalid" with a note that the document was modified after signing is the serious message. A DSC signature fingerprints the exact bytes of the file; any later edit - a corrected amount, an added page, even some innocent re-saves by unsuitable tools - breaks the fingerprint, deliberately by design.
What to do: do not rely on the document, and do not try to repair it. Go back to the signer and request a freshly signed copy of the corrected document, signed after the final edit. If the change is disputed, that broken status is itself important evidence - preserve the file exactly as received.
People say "but it showed a green tick" as if that settles everything. The tick (or its absence) only reports the cryptographic state: certificate trusted, document unchanged. It says nothing about whether the signer was the right person for this document, whether they had authority to sign it, or whether the contents are true. Conversely, the absence of a tick on an image-signed everyday form proves nothing bad - there was no certificate to tick.
Use the reader's verdict for what it is - a tamper and identity check for DSC files - and your own judgement for everything else. Confusing the two is how both false alarms and false confidence happen.
Watch the video to see the free signing route - and use this page whenever a receiver (or you) meets a signature warning on the result.
▶️ Watch Full VideoAlmost certainly not. The warning appears because the file has no certificate signature to validate - expected for a signature image. Explain honestly that it is an electronic signature image; if the receiver needs a verifiable signature, the document must be signed with a DSC instead.
Both - the difference is the reader, not the document. The second computer's reader does not yet trust the issuing authority. Updating its trusted certificate list (or following local IT policy for trust) normally turns the same file to valid without the signer doing anything.
No. The tick belongs to certificate validation, and an image carries no certificate. Any tool or person claiming their pasted image will "show verified" is misunderstanding or misrepresenting how PDF signatures work.