What is a .SIG File and How to Sign Documents with Electronic Signature Online

Received a contract or invoice along with a mysterious file named document.pdf.sig? In this guide, we explain what a .sig file is, how detached cryptographic signatures work, and how you can sign your own documents online directly in your browser.

What is a Detached Digital Signature (.SIG)?

A file with the .sig extension is an electronic digital signature file generated via cryptographic software (like PKCS#7 / CMS standards). It does not contain the readable text of the contract itself, but rather a cryptographic hash and the public certificate of the signer.

Visual Signature vs Cryptographic .SIG Signature

• Visual Facsimile / On-Page Signature: Embedded directly onto the PDF document surface for routine invoices, internal agreements, and NDAs.
• Cryptographic .SIG file: Used for official court filings, government portals, and commercial tenders requiring strict legal non-repudiation.

How to Open and View a .SIG File?

Since a .SIG file is binary cryptographic data, opening it in standard Notepad displays random unreadable symbols. To verify who signed it and whether the document was altered, you use official certificate validation portals (like Gosuslugi verification portal in Russia).