FeaturesToolsHelp
HomeToolsDigital Sign

Digital Signature for PDFs with PKI Certificates

Add cryptographically secure digital signatures to PDF documents using your PKCS#12 certificate (.pfx/.p12). PAdES-compliant signatures provide legal validity, tamper detection, and verifiable identity authentication through certificate-based cryptography.

Use Digital Sign NowBrowse All Tools

Key Features

Upload PKCS#12 certificates (.pfx or .p12 files)
Certificate validation with password verification
View certificate details: Common name, organization, email, validity period
PAdES-B-B compliant signatures (basic long-term validation)
Signature reason, location, and contact information fields
Position signature precisely: Bottom-right, bottom-left, top-right, center-bottom, or custom coordinates
Custom signature box size (width and height in points)
Visual position preview on PDF thumbnail
Page selection to sign specific pages
Tamper detection - any changes after signing invalidate the signature
Legally valid signatures recognized by Adobe Acrobat and other PDF readers
Secure transmission - certificates sent via encrypted HTTPS connection and never stored persistently

How to Digitally Sign a PDF

1

Upload your PDF file that requires a digital signature.

2

Click 'Choose Certificate' and upload your PKCS#12 certificate file (.pfx or .p12). These certificates are typically issued by Certificate Authorities or your IT department.

3

Enter the certificate password to unlock the private key.

4

Click 'Validate Certificate' to verify the password and load certificate information. You'll see the certificate owner's name, organization, email, and validity period.

5

Fill in signature details: Reason for signing (e.g., 'Document approval'), optional location (e.g., 'New York, NY'), and optional contact info.

6

Select the page number where the signature should appear.

7

Choose signature position: Select a preset (bottom-right, bottom-left, top-right, center-bottom) or use custom coordinates with percentage sliders for precise placement.

8

Adjust signature box size (width and height) to fit your document layout. Preview shows exact position on the PDF page.

9

Click 'Apply Digital Signature' to cryptographically sign the PDF with your certificate.

10

Download the signed PDF. The signature includes timestamp, signer identity, and tamper seal. Any modification after signing will invalidate the signature.

When You Need Digital Signatures

Sign legal contracts requiring cryptographic verification and non-repudiation
Execute financial documents, loan agreements, and banking forms with legal binding
Sign government forms, tax documents, and regulatory submissions
Approve corporate resolutions, board minutes, and executive documents
Sign intellectual property agreements, patents, and copyright registrations
Authenticate medical records, prescriptions, and HIPAA-compliant documents
Execute real estate contracts, deeds, and mortgage documents
Sign audit reports, financial statements, and compliance documents
Approve research papers, academic theses, and dissertations requiring author verification
Sign insurance policies, claims, and underwriting documents
Execute procurement contracts, vendor agreements, and RFPs requiring legal verification
Approve architectural plans, engineering drawings, and technical specifications in PDF format

Why Choose This Tool?

Legal validity - PAdES-compliant signatures are recognized globally for legal documents
Tamper-proof - any changes to the PDF after signing invalidate the signature
Identity verification - certificate proves signer's identity through trusted Certificate Authority
Non-repudiation - signer cannot deny signing the document
Timestamp proof - signature includes signing time for audit trails
Verifiable in Adobe Acrobat, Foxit Reader, and other PDF tools
Meets eIDAS (EU), ESIGN Act (US), and ZertES (Switzerland) standards
Secure processing - certificates transmitted via encrypted HTTPS and never stored on our servers

Frequently Asked Questions

What is a digital signature and how is it different from a visual signature?
A digital signature uses cryptographic certificates (PKI) to mathematically sign the document, providing tamper detection, identity verification, and legal validity. It's verifiable by PDF readers and recognized by courts. A visual signature (Sign PDF tool) is just an image placed on the document - it looks like a signature but has no cryptographic security or legal binding.
What is a PKCS#12 certificate and where do I get one?
PKCS#12 (.pfx or .p12) is a file format containing your digital identity - a private key and certificate. You can obtain one from: 1) Certificate Authorities like DigiCert, GlobalSign, or Sectigo (paid), 2) Your organization's IT department for internal documents, 3) Government agencies for official documents, or 4) Create a self-signed certificate for testing (not for legal documents).
How do I validate my certificate?
After uploading your certificate file, enter the password that protects the private key. Click 'Validate Certificate' to verify the password is correct. If valid, the tool displays certificate information: owner name, organization, email, and expiration date. If validation fails, check your password or certificate file.
What does 'PAdES-B-B' mean?
PAdES (PDF Advanced Electronic Signatures) is an EU standard for digital signatures. B-B (Basic) is the baseline level providing signature integrity and authenticity. Higher levels like B-T (Timestamp), B-LT (Long-Term), and B-LTA (Long-Term Archival) add timestamp authorities and long-term validation - these are planned features.
Can I use a self-signed certificate?
Yes, you can create and use self-signed certificates for testing or internal documents. However, they are not trusted by default in PDF readers and won't provide legal validity. For legally binding signatures, use certificates from trusted Certificate Authorities recognized globally.
What happens if I modify the PDF after signing?
Digital signatures include a tamper seal. Any change to the PDF after signing - even adding a single character - will break the signature. PDF readers like Adobe Acrobat will show a warning that the signature is invalid or the document has been altered. This protects document integrity.
Is my certificate password secure?
Yes. Your certificate and password are transmitted to our backend signing service via encrypted HTTPS connection for cryptographic operations. We do not store certificates or passwords persistently on our servers - they are used only for the signing operation and immediately discarded after processing. The signed PDF is returned to you and the certificate data is removed from memory.
Can I verify the signature after signing?
Yes! Open the signed PDF in Adobe Acrobat Reader or other PDF viewers. They will show a signature panel with verification status, signer identity, signing time, and whether the document has been altered. Valid signatures show a checkmark; invalid ones show a warning.
What's the difference between signature reason, location, and contact?
These are metadata fields embedded in the signature: 'Reason' explains why you signed (e.g., 'Approved', 'Author'), 'Location' indicates where you signed (e.g., 'New York, NY'), and 'Contact' provides contact info (e.g., email). Only 'Reason' is required; others are optional but useful for record-keeping.
Can I add multiple digital signatures to one document?
Yes. Download the signed PDF, then upload it again and add another signature. Each signer can add their own digital signature with their own certificate. PDF readers will show all signatures and verify each independently.
Will my signature work internationally?
PAdES signatures are recognized globally. The EU eIDAS regulation, US ESIGN Act and UETA, and many other jurisdictions accept digital signatures as legally binding. However, specific legal requirements vary by country and document type - consult legal advice for critical documents.
What if my certificate is expired?
The tool will reject expired certificates. Digital signatures require valid certificates at signing time. If your certificate is about to expire, renew it before signing documents. Already-signed documents remain valid even after the certificate expires, as long as they were signed while the certificate was valid.
Can I position the signature anywhere on the page?
Yes. Use preset positions (bottom-right, bottom-left, top-right, center-bottom) for quick placement, or choose 'Custom Position' and use X/Y sliders to position the signature box precisely. The preview shows exactly where the signature will appear on the PDF page.

Related Tools

Sign PDF

Add visual signature to PDF

Encrypt PDF

Password protect PDF

Decrypt PDF

Remove PDF password

Ready to Get Started?

Fast PDF processing with our powerful online tool. Works entirely in your browser, no installation needed.

Try Digital Sign FreeExplore All Tools
Secure Processing
256-bit SSL encryption
Lightning Fast
Process files in seconds
Free to Start
Start with free tier