Fix "Digital Signature Is Invalid" PDF Error
"Digital signature is invalid" in Adobe Reader can mean tampering, an expired certificate, or a configuration issue. Here's how to diagnose which applies to your document.
"The document has been altered or corrupted since it was signed" or simply "Signature is invalid" — this error in Adobe Reader is one of the more alarming PDF messages you can encounter. But "invalid" doesn't always mean what people fear. This guide walks through every reason a valid signature shows as invalid and how to resolve each.
What "Invalid" vs "Unknown" Signature Means
Adobe Reader shows three distinct signature states that are often confused. "Valid" (green checkmark) means the document hasn't changed since signing and the certificate chain is trusted. "Unknown" (question mark icon) means Reader can't verify the certificate — usually because the signing authority isn't in Reader's trust list — but the document hasn't necessarily been altered. "Invalid" (red X) means the cryptographic hash doesn't match the document — the file content changed after signing. Only "Invalid" indicates a genuine problem; "Unknown" is usually a certificate trust issue.
Fix 1 — Add the Signing Certificate to Trusted Identities
If the signature shows as "Unknown" (not "Invalid"), the issue is that Adobe doesn't trust the certificate authority, not that the document was altered. In Adobe Reader: click the signature → Signature Properties → Show Signer's Certificate → Trust tab → Add to Trusted Identities. Check "Use this certificate as a trusted root." Validate the signature again. This is commonly needed for government-issued PDF signatures, corporate PKI signatures, and signing certificates from non-US certificate authorities not pre-installed in Adobe's trust store.
Fix 2 — Check If the Certificate Has Expired
Digital signing certificates are issued with validity periods — typically 1-3 years. After the certificate expires, Adobe marks the signature as invalid even if the document itself was signed while the certificate was valid. Click the signature → Signature Properties → Show Signer's Certificate and check the "Valid From" and "Valid To" dates. An expired certificate on an old document doesn't mean the document was altered — it means the certificate expired after signing. Adobe Acrobat Pro can be configured to validate signatures at the time of signing rather than at the current date.
Fix 3 — Remove and Re-apply the Signature
If you created the PDF and the signature is showing as invalid because the document changed after signing (which invalidates the hash), you need to clear the signature and re-sign. This is expected behaviour — even a whitespace change in the file after signing breaks the cryptographic proof. In Acrobat Pro: right-click the signature field → Clear Signature. Make all necessary changes to the document, then sign again using FixMyPDF's signing tool or Acrobat Pro.
Fix 4 — Check If the PDF Was Re-saved After Signing
A common mistake: someone receives a signed PDF, opens it in a PDF editor, and saves it — even without making visible changes. Many PDF editors (not Adobe Reader) make invisible structural changes during save (compressing objects, updating timestamps) that break the signature hash. The fix is to re-obtain the original signed version from the source. Going forward, when you receive a signed PDF that you need to archive, store it immediately without opening in any editing application.
When "Invalid" Means Actual Tampering
If a contract, certificate, or official document shows as "Invalid" and the signer confirms they haven't re-saved the file, the document may have been altered after signing. This is the scenario digital signatures exist to detect. Do not rely on or act on the document's contents without verifying authenticity through an independent channel — contact the signer directly to request a fresh copy. In high-stakes situations (legal documents, financial instruments), engage a document forensics specialist.
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