Certificate Sign tool allows you to sign PDFs using a digital certificate (X.509), providing a higher level of security and authenticity compared to visual signatures. This makes it suitable for legal documents, official forms, contracts, or any document that requires verified digital signatures. The tool supports standard certificate formats (.pfx and .p12) and allows you to add certificate information including reason for signing and location. Digital certificates provide cryptographic proof of identity and document integrity, making them legally binding in many jurisdictions. The certificate signature is embedded directly into the PDF, creating a tamper-evident seal that shows if the document has been modified after signing. All processing happens locally in your browser, ensuring your certificate and private keys never leave your device. The resulting certificate-signed PDF maintains high quality and can be downloaded immediately after signing. Perfect for legal professionals signing contracts, government workers signing official documents, or anyone who needs verified digital signatures on their PDFs.