How To Validate a Digital Signature in a PDF

When you receive a signed PDF, you want to know one thing: Can you trust it? 

Validating a digital signature tells you who signed the document, when they signed it, and if anything has changed since. If something looks off, you can catch problems before you approve, pay, or sign anything yourself.

You can validate a digital signature in PDF with standard PDF readers. We will walk you through the process, explain common warnings, and show where Smallpdf eSign and Sign.com fit in.

Before you start clicking on panels and certificates, it helps to understand what you are checking.

How Digital Signatures Work

A digital signature is more than a scribble on a screen. It combines:

• A cryptographic key that belongs to the signer

• A unique fingerprint of the document content

• A timestamp that records when the signature was applied

When someone signs a PDF digitally, the software:

1. Creates a fingerprint of the document.

2. Encrypts that fingerprint with the signer’s certificate.

3. Embeds the result in the PDF as a digital signature.

If the PDF changes later, the fingerprint no longer matches, and the signature shows as invalid.

Why You Should Validate Every Signed PDF

Validation helps you:

• Confirm that the document has not been modified after signing.

• Check that the signer’s certificate is trusted and still valid.

• Understand if the signature meets your legal or compliance needs.

Instead of guessing from a checkbox or a name typed on a line, you get a clear status you can rely on.

Here’s how to check if a digital signature is valid using Adobe Acrobat Reader or another signature-aware PDF viewer.

Step 1: Open the Signed PDF in a Compatible Viewer

Open your PDF in Adobe Acrobat Reader or another PDF viewer that supports digital signature validation.

Make sure you are using a recent version so the certificate checks work correctly.

Step 2: Locate the Signature Status Area

Look for a signature bar or panel:

• In Adobe Acrobat Reader, you’ll usually see a signature status bar at the top.

• You can also open the “Signatures” panel from the left-hand sidebar (the fountain-pen icon) or via “View > Show/Hide > Navigation Panes > Signatures.”

Step 3: Open the Signatures Panel

In the Signatures panel:

• Click “Signatures” to expand it.

• You’ll see a list of all digital signatures in the document.

• Select the signature you want to review.

Step 4: View Signature Properties

Right-click the selected signature (or use the panel options) and choose “Show Signature Properties…”

In the properties window, check:

• Signing time: When the document was signed

• Signer identity/certificate: Who signed and which certificate was used

• Document status. Whether the document has been changed since it was signed

Step 5: Check the Validation Status

Look at the status indicators:

• A “green checkmark” usually means the signature is valid, the certificate is trusted, and the document has not been modified.

• A “yellow warning triangle” indicates a problem, such as an untrusted certificate or incomplete validation.

• A “red X” indicates an invalid signature or a document that has been changed after signing.

Step 6: Review Any Warnings or Errors

After validation:

• Reopen the “Signature Properties” if needed.

• Read any warnings like expired certificate, revoked certificate, or document has been altered since signing.

If you see warnings or errors, treat the document as not fully trustworthy until you’ve verified the signer and the changes through another channel.

You can repeat these steps for each signature in the list if the PDF contains multiple signed revisions or multiple signers.

Sometimes you need more than a simple green checkmark, especially for legal, financial, or compliance-heavy documents.

1. Check Certificate Information

From the signature details, open the certificate view to see:

• The signer’s name or organization

• The issuing certificate authority (CA)

• The certificate’s validity period (start and end dates)

• Any extra information about the signer’s identity

If you trust the signer and their CA, you can often mark them as trusted so future signatures from them validate more smoothly.

2. Verify the Timestamp

A digital timestamp proves when the document was signed. This can matter when:

• A certificate has expired, but was valid at signing time.

• Legal deadlines or contract windows depend on exact dates.

Check that:

• The timestamp falls inside the certificate’s validity period.

• The timestamp server is trusted or recognized by your system.

If the timestamp is valid and the document has not changed, the signature usually remains valid even if the certificate later expires.

Things don’t always go perfectly. Here are typical issues and how to handle them.

1. Signature Shows “Validity Unknown” or a Question Mark

This usually means the viewer cannot confirm the certificate chain or has not checked it yet.

Try these steps:

• Click “Validate signatures” or the equivalent option.

• Make sure you are online so the viewer can reach the certificate authority.

• Check if your system trusts the CA that issued the signer’s certificate.

If the certificate comes from an internal or private CA, you may need your IT team to add it to the trusted list.

2. Certificate Is Expired or Revoked

When you see warnings about an expired or revoked certificate:

• Look at the signing time and compare it with the certificate’s valid dates.

• If the document was signed while the certificate was valid, the signature may still be acceptable in your context.

• If the certificate was already revoked before signing, treat the signature with caution and confirm directly with the signer.

Always follow your organization’s policy for expired or revoked certificates.

3. Document Was Modified After Signing

If the viewer reports that the document has been modified after signing, treat this as a serious warning.

In this case:

• Do not rely on the current version for critical decisions.

• Contact the signer and request a fresh, unmodified copy.

• Compare the modified file with the original if you have an earlier version.

A valid digital signature should confirm that nothing has changed since signing. If that guarantee is gone, you should pause and verify.

4. Viewer Cannot Validate the Signature

Sometimes the issue is not the signature, but the software.

If your viewer shows limited information or no validation options:

• Open the PDF again for a clearer view of the signature status.

• Check for software updates if you use a desktop reader in parallel.

• Avoid relying on simple browser previews for serious validation, as they often do not support full digital signature checks.

Using a reader that understands digital signatures is crucial if you depend on them for compliance.

Once you know how to validate signatures, the next step is creating signed PDFs that others can trust.

Sign PDFs With Smallpdf eSign

With Smallpdf eSign, you can:

• Upload a PDF and add signature fields where needed.

• Create your own reusable signature.

• Invite others to sign in with a clear, guided flow.

• Download a signed PDF that can be validated later.

This works well for everyday agreements, approvals, and internal workflows where you need a reliable, user-friendly signing experience.

Use Sign.com for Advanced Digital Signing

For more advanced digital signing needs, you can also use our dedicated solution at Sign.com. It offers:

• Stronger identity proofing options

• More advanced certificate and signature management

• Features tailored to teams with heavier signing and compliance requirements

You can start with Smallpdf eSign for simple workflows, then move to Sign.com if your needs grow.

You can try eSign out for free to see if you like the workflow, the signature quality, and the overall usability.

And if you’re tempted to upgrade your signatures from electronic to digital, we always offer a 7-day free trial for you to get started!

Smallpdf helps you get the PDF into a format where page-size fixes are easier to make. Convert the file to Word, Excel, PowerPoint, or JPG, depending on what the PDF was originally built from. Then adjust the page size, orientation, scaling, or margins in the matching editor before saving it back as a PDF.

Once the corrected PDF is ready, a few other Smallpdf tools can help tidy up the final file. Use Merge PDF if you fixed separate documents and need to combine them again. And use Compress PDF if the finished file is too large to email or upload.

If the mixed-size pages came from scanned documents or images, you may also find Crop PDF useful before the final conversion. Trimming extra white space can make pages look more consistent before you rebuild the PDF.

Free accounts include a limited number of files each day. If you work with PDFs often, Pro removes that daily limit, gives you access to extra features, and includes a 7-day free trial.