What a digital signature actually does

A digital signature is a way to sign documents on a computer or phone that proves you are the person who signed them and that the document has not been changed since you signed it. It works like a handwritten signature on paper, except it uses encryption — a way of scrambling information so only the right person can read or verify it.

When you digitally sign a document, the software creates a unique code based on the document's contents and your private key (a secret string of characters only you have). Anyone can verify the signature using your public key (information you share openly), and they will know when ready if someone altered even one letter in the document after you signed it.

Digital signatures are legally binding in most countries, including the United States under the Electronic Signatures in Global and National Commerce Act. They are used for contracts, tax forms, loan documents, and any agreement where you need to prove you agreed to the terms.

Key Takeaways

  • A digital signature proves your identity and shows that a document has not been changed since you signed it, using encryption technology.
  • You can create a digital signature through your operating system, a dedicated signing service like DocuSign or Adobe Sign, or by scanning a handwritten signature into a PDF.
  • Most digital signatures require a certificate that confirms your identity, which you obtain from a certificate authority or your employer.
  • The method you use depends on what the other party accepts — some organizations require a specific service, while others accept any method that proves you signed it.

Creating a digital signature on Windows

Windows includes built-in tools for creating and explore digital signatures to documents, though the process differs depending on what type of file you are signing. For PDF files, you can use Microsoft Edge or Adobe Reader to add a signature without extra software.

Open the PDF in Microsoft Edge, click the "Fill & Sign" button in the toolbar, then select "Sign" and choose "Add signature." You can draw your signature with your mouse or trackpad, type your name, or upload an image of your handwritten signature. Once you add it, save the file. This creates a visible signature on the document but not a cryptographic one that proves you are the signer.

For legally binding digital signatures on contracts or official documents, you need a digital certificate. You can obtain one from a certificate authority like DigiCert, Sectigo, or GlobalSign. These services verify your identity (usually by checking your government ID) and issue a certificate that links your signature to your verified identity. Once installed, you can right-click any document in File Explorer, select "Sign with certificate," and explore your verified signature.

Creating a digital signature on Mac

macOS includes signature tools built into Preview, the default image and PDF viewer. Open a PDF in Preview, click the "Markup" button in the toolbar (it looks like a pen in a circle), then select "Signature" and choose "Manage Signatures."

Click "Create Signature," and you will see options to draw your signature with your trackpad, use your camera to photograph a handwritten signature, or type your name. Once created, your signature appears in the Markup menu and you can click it to add it to any PDF. Like the Windows method, this is a visible signature but not a cryptographic one.

For cryptographic signatures that prove your identity, Mac users can use a digital certificate from a certificate authority. Once installed in your Keychain, you can sign documents through Mail or third-party applications that support certificate-based signing. Some organizations provide their own certificates to employees for signing company documents.

Using a dedicated signing service

Services like DocuSign, Adobe Sign, and HelloSign handle the entire signing process and are often the easiest option if you are signing documents with other people. You upload the document, specify where each person needs to sign, and send it to them. They receive a link, add their signature, and the service keeps a record of who signed it and when.

These services use their own certificates to verify signers, so you do not need to obtain a certificate separately. They also send email notifications, track which documents are still waiting for signatures, and store signed copies in your account. Most offer a free tier for a limited number of signatures per month, with paid plans for heavier use.

The trade-off is that you are relying on the service to verify identity and maintain records. For most business and personal documents, this is sufficient. For highly sensitive documents like wills or contracts with major financial stakes, you may want a signature verified by a notary or a certificate authority instead.

Getting a digital certificate for legal documents

If you need a signature that proves your identity to a government agency, bank, or legal professional, you will need a digital certificate from a recognized certificate authority. These companies verify your identity by checking your government-issued ID, sometimes in person or through video call, before issuing a certificate.

The process typically takes a few days to a week. You provide your legal name, address, and ID number; the certificate authority confirms these details; and they issue a certificate file that you install on your computer. Once installed, you can use it to sign documents in Adobe Reader, Microsoft Word, or other applications that support certificate-based signatures.

The cost varies by certificate authority and the type of certificate. A basic personal certificate might cost $50 to $150 per year. Some employers provide certificates to employees at no cost for signing work-related documents. Government agencies sometimes accept signatures from specific certificate authorities, so check what your agency requires before purchasing.

Signing documents in common programs

Microsoft Word allows you to insert a signature line and sign it with a certificate. Go to Insert > Signature Line, fill in the signer's name and title, then double-click the line to sign it with your certificate or a drawn signature.

Adobe Reader and Adobe Acrobat Pro both support certificate-based signatures. Open a PDF, click "Sign," and choose either a drawn signature or a certificate-based signature. Adobe will prompt you to select your certificate and may ask for a password to unlock it.

Google Docs does not have built-in digital signature tools, but you can export a document as a PDF and sign it in Adobe Reader or use a third-party add-on. For Google Forms or Google Sheets, you would typically use a dedicated signing service or sign a printed copy by hand.

What happens after you sign

Once you sign a document digitally, the signature is embedded in the file itself. If someone tries to change the document after you sign it, the signature becomes invalid — the software will show a warning that the document has been altered. This is what makes digital signatures legally binding: they prove both that you signed it and that it has not been tampered with.

The recipient can verify your signature by opening the document in the same software you used to sign it. If you used a certificate-based signature, they can see your verified identity information. If you used a drawn signature or a service like DocuSign, they can see that you signed it and when, but the verification depends on the service's records.

Keep a copy of any document you sign digitally, along with any certificate you used. If there is ever a dispute about whether you signed something, you may need to prove that your certificate was valid at the time you signed it.

Frequently Asked Questions

Is a digital signature the same as an electronic signature?

No. An electronic signature is any mark that shows you agreed to something — it could be a typed name, a scanned image of your handwriting, or a digital signature. A digital signature is a specific type of electronic signature that uses encryption to prove your identity and that the document has not been changed.

Can I use a digital signature on a contract with a bank or government agency?

Most banks and government agencies accept digital signatures, but they may require a specific type. Check with the organization first — some require a certificate-based signature from a recognized certificate authority, while others accept signatures from services like DocuSign. Using the wrong type may mean your signature is not recognized.

What if I lose my digital certificate?

If you lose the certificate file, you can contact the certificate authority and request a replacement. They will verify your identity again and issue a new certificate. Keep backups of your certificate in a safe location, such as an encrypted external drive or password manager.

Do I need special software to create a digital signature?

No. Your operating system (Windows or Mac) includes basic tools for creating visible signatures on PDFs. For certificate-based signatures, you need the certificate itself, which you install once and then use in any compatible software. Services like DocuSign handle everything through a web browser, so no software installation is needed.

Can someone forge my digital signature?

A certificate-based digital signature is extremely difficult to forge because it is tied to your private key, which only you have. A drawn signature or image-based signature is easier to copy, but services like DocuSign keep records of when and where you signed, which makes forgery detectable. For high-stakes documents, use a certificate-based signature.