How to Detect Whether a PDF Was Altered After Signing
Question details
The user needs to know how to verify the integrity of a signed PDF document to ensure it has not been modified or tampered with by third parties.
- Product
- Device & OS
- not provided
- Scenario
- Auditing or verifying the authenticity and integrity of a signed PDF document to ensure no unauthorized changes were made.
- Observed behavior
- Determining the validation status of a document to see if post-signing edits have invalidated the digital signature.
Ensure you have a PDF reader capable of validating digital certificates and signatures. You will also need the digitally signed PDF file in question to perform the verification.
Verify Digital Signature Status
The most reliable way to detect alterations is by checking the digital signature panel. Any modifications made after signing will automatically invalidate the signature.
When a document is digitally signed, it uses cryptographic techniques to bind the document's exact state to the signature. If even a single character or element is altered, the PDF software will break the hash, flagging the signature as invalid or indicating that the document was modified post-signing.
Launch your PDF reader and open the digitally signed PDF file you wish to verify.
Look for a signature notification bar at the top of the document, or click on the fountain pen or rosette icon on the left sidebar to open the 'Signatures' panel.
Review the signature status message. If it displays 'Signature is valid' and 'Document has not been modified since this signature was applied,' the file is unaltered.
If the panel displays 'Signature is invalid' or 'Document has been modified since it was signed,' it confirms unauthorized edits have been made to the file.
Compare with Previous Signed Versions
If the PDF was signed multiple times or saved with revisions, you can view the exact state of the document at the time of signing to see what was changed.
Use WPS Office to Digitally Sign and Validate PDFs
WPS Office provides robust PDF security features, allowing you to easily apply, view, and validate digital signatures to protect your documents from unauthorized alterations.
- 1. Open Your PDF: Launch WPS Office and open the PDF document you want to verify or sign.
- 2. Access Protection Tools: Navigate to the 'Protect' tab on the top ribbon menu.
- 3. Validate Signatures: Click on the digital signature field within the document to automatically run a validation check on its cryptographic integrity.
- 4. Sign New Documents: Click on 'Sign Document' to apply a new digital certificate to safeguard your file against future edits.
Frequently Asked Questions
Can I detect changes in a PDF that does not have a digital signature?
No. Without a digital signature, a trusted revision history, or an original copy to compare against, standard PDF software cannot reliably detect or prove that a PDF has been altered.
What happens to a digital signature if I edit the PDF text?
Any edit to the content of a digitally signed PDF, such as changing text or adding images, breaks the cryptographic hash. The PDF software will immediately flag the signature as 'Invalid' or state that the document has been modified.
Are electronic signatures the same as digital signatures for detecting alterations?
No. Electronic signatures (like a typed name or a drawn image of a signature) do not offer cryptographic protection. You must use a certificate-based digital signature to securely detect document alterations.
Can I see exactly what was changed after the document was signed?
Yes. Advanced PDF readers allow you to right-click the signature and select 'View Signed Version.' You can then compare the signed version with the current version to see the specific alterations made.
