You receive a contract, a financial statement, or an official document as a PDF. How do you know it hasn't been modified since it was created or last signed? The answer depends on how the document was protected when it was issued. Some PDFs provide strong, verifiable integrity guarantees. Others provide none at all.
Digital Signatures: The Only Reliable Integrity Check
A certified digital signature creates a mathematical fingerprint of the document at the moment of signing. If any byte of the document changes after the signature is applied โ a character edited, a number changed, even a space added โ the signature becomes invalid. This is cryptographic tamper-evidence: the math either checks out or it doesn't.
To check a digital signature in Adobe Reader: open the document and look for the signature panel on the left side, or a blue information bar at the top indicating signature status. Click on the signature to see its validity. "Signature is valid" means the document hasn't been altered since signing. "Signature is invalid" or "Document has been modified" means something changed after the signature was applied โ treat the document with significant skepticism.
What If the Document Has No Digital Signature?
Most PDFs in everyday use โ contracts, invoices, reports โ don't have cryptographic digital signatures. They may have an image of a signature, a stamp, or a watermark, but none of these provide tamper-evidence. An image of a signature is just pixels; changing the document content around it is trivial for anyone with a PDF Editor.
For unsigned PDFs, there's no technical method to definitively prove the document hasn't been altered. What you can do is look for signs of modification and compare with other evidence.
Signs a PDF May Have Been Altered
While none of these are conclusive, they raise questions worth investigating:
- Inconsistent fonts: text that looks slightly different from surrounding content โ different weight, different spacing, or slightly different letterforms โ may have been inserted using a different font or a different PDF editor
- Inconsistent metadata: check File > Properties > Description. If the "Modified" date is significantly later than the "Created" date, the document was edited after creation. A modification date of today on a document dated years ago is a red flag.
- Multiple PDF creation tools: the Creator and Producer fields in Properties may list different applications, suggesting the document was processed by more than one tool after initial creation
- Visual inconsistencies: zoom in to 200-400% on suspicious areas. Text that was pasted over existing content sometimes has subtle alignment, spacing, or rendering differences that are invisible at normal zoom
Checking PDF Metadata for Modification Evidence
PDF metadata records creation and modification dates, the software used to create it, and sometimes the author's name. In Adobe Reader or Acrobat, go to File > Properties and check the Description tab. Also click Additional Metadata for the full XMP metadata which may contain more detailed history.
Metadata can be edited โ a sophisticated alteration would include updating the metadata to remove evidence of modification. But many alterations are done hastily and the metadata is left unchanged, providing a simple first check. A document claiming to be an invoice from two years ago with a modification date of yesterday warrants scrutiny.
How to Protect Your Own Documents Against Alteration
If you're issuing documents that others need to trust, use a certified digital signature. Adobe Acrobat Pro supports PDF digital signatures using a digital certificate. For business use, certificates from trusted Certificate Authorities (DigiCert, GlobalSign, and others) provide third-party verified identity alongside the tamper-evidence.
For documents that don't require cryptographic signatures, applying PDF Security permissions that restrict editing provides a softer deterrent โ it doesn't prevent determined modification but makes accidental or casual changes less likely. Combining a permissions restriction with a visible certification statement ("This document is certified by [Organization]") makes the intended official status clear to recipients.
The Honest Reality
Without a verified digital signature, there's no technical guarantee a PDF is unaltered. Metadata checks and visual inspection raise or lower suspicion but don't prove integrity. For documents where authenticity is legally or financially critical โ contracts in dispute, financial statements used for lending, official certificates โ the presence or absence of a valid digital signature is the only reliable indicator. Everything else is circumstantial evidence.