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SECURE
Sign a PDF with your PKCS#12 certificate, so anyone can check who signed it and that it hasn’t changed since.
QUICK ANSWER
To digitally sign a PDF, add the file, choose your .p12 or .pfx certificate and type its password. Add a reason, location and name if you like, then choose Sign with certificate. The signature is applied in your browser, and the certificate and password never leave your device.
Updated 31 October 2026 · Written by the StackPDF teamUpdated 31 October 2026 · StackPDF team
You’ll choose your certificate next. It stays in this browser.You’ll choose your certificate next. It stays in this browser.
PDF files · Runs in your browser
QUICK ANSWER
To digitally sign a PDF, add the file, choose your .p12 or .pfx certificate and type its password. Add a reason, location and name if you like, then choose Sign with certificate. The signature is applied in your browser, and the certificate and password never leave your device.
Updated 31 October 2026 · Written by the StackPDF teamUpdated 31 October 2026 · StackPDF team
HOW TO
Drop it here, or choose it from your computer, your phone or My files.
Pick your .p12 or .pfx file and type its password. Add a reason and location if you need them.
Viewers such as Adobe Acrobat show who signed it and whether it has changed.
GOOD TO KNOW
Both are called signing, but they do different jobs.
| TYPE | WHAT IT IS | USE IT WHEN |
|---|---|---|
| Digital signature | A cryptographic seal from your certificate | You need proof of who signed and that nothing changed |
| Drawn or typed signature | A picture of your signature on the page | Someone asks you to sign a form or a letter |
| Both | A drawn signature, then a certificate seal | A contract asks for both |
Sign the PDF last: any change after signing breaks the signature. Use Sign PDF for a drawn signature.
PRIVACY
Yes. Your certificate and its password are used in your browser to sign the file, and are never sent to StackPDF.
The .p12 or .pfx file is read in your browser and never uploaded.
It’s used once to unlock the certificate, then discarded.
Signing uses pdf-lib, node-forge and @signpdf, inside your browser.
FAQ
Add the PDF, choose your .p12 or .pfx certificate, type its password and choose Sign with certificate. You download the signed PDF.
Yes. The free plan includes 1 PDF export a day, with no account. Premium removes the daily limit for $5.99 a month or $39.99 a year.
No. The certificate and its password are used in your browser to sign the file, and neither is sent to StackPDF.
From a certificate authority, your company’s IT team or, in some countries, a national e-ID service. Ask for a document signing certificate as a .p12 or .pfx file.
Acrobat only trusts certificates from authorities on its trust list. A self-made certificate, or one from an authority it doesn’t know, shows as unknown until the reader chooses to trust it.
Yes. Verify PDF signatures lists the signatures in a PDF, with the signer, date and certificate of each one.