FormatShiftWhat is a .SIG File, How to Open It, and How Digital Signatures Work
Received a contract or invoice with a companion file named document.pdf.sig? In this guide, we explain what a .sig file is, why it cannot be opened like a standard document, how to verify it online, and how visual signatures differ from cryptographic signatures.
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What is a Detached Digital Signature (.SIG)?
A file with the .sig extension typically contains digital signature data associated with a primary document. In detached signatures, the signed document itself is stored separately, while the .sig container holds the cryptographic data and signer certificate information needed for verification.
If the original document is modified after signing, cryptographic verification will fail because the data hash no longer matches the signature.
How to Open a .SIG File
A .SIG file cannot be opened as a regular Word or PDF document. It contains binary cryptographic data rather than readable text. If you have a pair like contract.pdf and contract.pdf.sig, the actual agreement text is inside contract.pdf.
Quick Reference: Understanding What Files You Have
| What You Have | What It Means |
|---|---|
| document.pdf | The primary readable document. |
| document.pdf.sig | Detached cryptographic signature data for the document. |
| Only .sig file | Cannot read the contents; the original document is missing. |
| PDF with stamp/signature image | Visual signature (facsimile), not necessarily a cryptographic signature. |
| PDF + valid .sig file | Ready for full cryptographic verification using validation services. |
How to Verify a .SIG File Online
To verify that a detached signature is authentic and the document was not altered:
- Locate the original document (e.g.,
agreement.pdf). - Locate the corresponding signature file (e.g.,
agreement.pdf.sig). - Open a certified cryptographic validation portal or desktop crypto utility.
- Upload both files together as prompted.
- Review the verification report showing signer identity and certificate validity.
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Go to Document Signer →Visual Signatures vs Cryptographic .SIG Signatures
It is crucial to understand that a visual signature and a cryptographic .SIG file serve different purposes:
- Visual Signature: An image of a handwritten signature or stamp placed onto a PDF page. Useful for internal approvals, routine invoices, and mutual operational agreements.
- Cryptographic .SIG Signature: A dedicated container with mathematical signature data intended for technical verification of identity and document integrity.
Important: Placing a signature image on a PDF page does not by itself create a qualified cryptographic electronic signature.
Frequently Asked Questions
Can I open a .SIG file in Word or Notepad?
No. A .SIG file contains cryptographic signature data. To read the agreement, open the primary document (such as PDF or DOCX) with the corresponding filename.
Why are both .pdf and .sig files needed?
In detached signatures, the document and signature are separate. The verification tool needs the document to calculate its current checksum and match it against the signature file.
How do I convert a signed PDF to Word?
If you need to edit the document text, use our PDF to Word OCR Converter.