How to Sign a PDF Document With a Digital Signature

The basic process: what happens when you sign

When you add a digital signature to a PDF, you are attaching a unique encrypted code that proves you signed it and that the document has not been changed since. The steps are straightforward: open the PDF in software that supports digital signatures, click the signature field or create one, choose how to sign (with a certificate you already have, or by drawing your name), and save the file. The signature stays attached to the document and can be verified by anyone who opens it.

Most people sign PDFs using one of three methods. The first is a self-signed signature, which you create yourself in seconds and works fine for internal documents or situations where the other person knows you. The second is a certificate-based signature, which uses a digital certificate issued by a trusted company and proves your identity to strangers. The third is a cloud-based signature service, which handles the signing through a website or app and often includes tracking and legal documentation. Which one you use depends on what the document is for and who needs to trust it.

Key Takeaways

Self-signed signatures take 30 seconds and work for documents between people who already know each other, but do not prove your identity to strangers.
Certificate-based signatures require you to obtain a digital certificate from a company like DigiCert or Sectigo, which costs $50 to $300 per year and proves your identity.
Cloud services like DocuSign, Adobe Sign, and HelloSign handle the entire signing process online and include proof that you signed on a specific date and time.
Most PDFs you encounter will have a signature field already built in, but you can add one yourself if the document does not have one.
Once you sign a PDF, the signature is locked to that version of the document — if anyone changes the content, the signature becomes invalid.

Signing with software you already have

If you have Adobe Acrobat Reader (the free version) or Adobe Acrobat Pro, you can sign a PDF without buying anything extra. Open the PDF, click the "Sign" button in the toolbar, 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. Adobe will save this as your default signature so you do not have to recreate it each time.

For a self-signed signature in Adobe, you do not need a certificate. The signature appears on the document, and anyone can see that you signed it, but they cannot verify that you are actually who you claim to be — only that someone with access to your computer signed it. This is fine for internal documents, contracts between people who know each other, or anything where legal proof of identity is not required. If the document needs to be legally binding and you are signing for a stranger or a company, you will need a certificate instead.

On Windows, you can also use Microsoft Edge to sign PDFs. Open the PDF in Edge, click the "Fill & Sign" button, and choose "Sign." The process is the same as Adobe: draw, type, or upload your signature. On Mac, Preview (the built-in image viewer) lets you sign PDFs by clicking "Tools" and then "Annotate," though this method is less straightforward than Adobe or Edge.

Getting a digital certificate for legal documents

A digital certificate is a file issued by a trusted company that proves your identity. When you sign with a certificate, the recipient can verify that the signature came from you specifically, not just from someone with access to your computer. This matters for contracts, financial documents, or anything where the other party needs legal proof that you signed it.

To get a certificate, you purchase one from a certificate authority. Common providers include DigiCert, Sectigo, GlobalSign, and Entrust. The process usually takes a few days: you submit your information, the company verifies your identity (often by calling your phone number), and they issue a certificate file that you read and install on your computer. Costs range from $50 to $300 per year depending on the provider and the type of certificate.

Once you have the certificate installed, signing with it in Adobe Acrobat Pro is the same as a self-signed signature — you click "Sign," choose your signature appearance, and the software automatically uses your certificate. The difference is that when someone opens the signed PDF, they see your name and the certificate issuer, and they can verify that the signature is genuine. In Acrobat Reader, you can view and verify certificate-based signatures but cannot create them — you need the Pro version to sign.

Using cloud signature services for tracking and proof

Services like DocuSign, Adobe Sign, HelloSign, and Dropbox Sign handle signing through a website or app. You upload the PDF, add signature fields where they need to go, and send it to the other person. They sign online, and the service records the date, time, and IP address of the signature. The document is then locked and cannot be changed.

These services are useful when you need proof that someone signed on a specific date, when you are signing with multiple people, or when you want to avoid sending files back and forth by email. Most offer a free tier that lets you sign a few documents per month, with paid plans starting around $10 to $15 per month for individuals. DocuSign and Adobe Sign are the most widely used, but HelloSign and Dropbox Sign are simpler if you do not need advanced features.

To sign a document through one of these services, you receive an email with a link. You click the link, review the document, click where it says "Sign Here," and either draw your signature, type your name, or upload an image. The service then emails you a copy with a certificate of completion that shows when you signed. The original document is stored in the service's system, so you can retrieve it later if you need proof that you signed it.

Adding a signature field if the document does not have one

Some PDFs come with signature fields already in place — you just click the field and sign. Others do not. If you need to add a signature field yourself, you can do it in Adobe Acrobat Pro by clicking "Prepare Form," then clicking where you want the signature to go, and choosing "Signature Field" from the toolbar. Name the field and click "Done."

In cloud services like DocuSign or Adobe Sign, you can add signature fields by uploading the PDF and clicking where each signature should go. The service will add the field and send the document to the signer with instructions. You do not need to edit the PDF yourself — the service handles it.

If you are using free software like Adobe Acrobat Reader or Microsoft Edge, you cannot add signature fields — you can only sign in fields that already exist. If the document does not have a field and you need to add one, you will need either Adobe Acrobat Pro or a cloud service.

What happens after you sign

Once you sign a PDF, the signature is locked to that version of the document. If anyone changes the content — even one word — the signature becomes invalid. Most PDF readers will show a warning that says "This document has been modified" or "Signature is invalid." This is a security feature: it prevents someone from changing the contract after you signed it and claiming you agreed to the new version.

If you need to make changes to a signed document, you have two options. You can unsign it (in Adobe, right-click the signature and choose "Clear Signature"), make your changes, and sign again. Or you can create a new version, have everyone sign that version, and keep the old signed version as a record. For important documents, the second option is safer because it creates a clear audit trail.

After signing, save the PDF with a clear filename that includes the date — for example, "Contract_2024-01-15_signed.pdf" — so you know which version is the final signed one. If you signed through a cloud service, the service stores a copy automatically, so you do not have to worry about losing it.

Troubleshooting common signing problems

If you cannot find the "Sign" button in Adobe Acrobat Reader, you are using the free version, which has limited signing features. You can view and verify signatures, but to create a self-signed signature, you need either Adobe Acrobat Pro or a free alternative like Microsoft Edge or a cloud service.

If you see a message that says "This signature is not trusted," it usually means the signature is self-signed and the recipient's computer does not recognize it as coming from a trusted source. This is normal for self-signed signatures. If you need the signature to be trusted, you will need a certificate from a certificate authority.

If the signature field is not clickable or does not appear, the PDF may have been locked by the creator. Some PDFs are set to prevent editing or signing. If this happens, you can ask the document creator to unlock it, or you can use a cloud service like DocuSign, which can add signature fields to locked PDFs.

Frequently Asked Questions

Can I sign a PDF on my phone?

Yes. Cloud services like DocuSign, Adobe Sign, and HelloSign all have mobile apps that let you sign PDFs on iPhone or Android. You can also use Adobe Acrobat Reader on mobile to sign PDFs with a self-signed signature by drawing your name on the screen. The process is the same as on a computer.

Is a digital signature legally binding?

A self-signed signature is legally binding in most situations, but it does not prove your identity — only that someone with access to your computer signed it. A certificate-based signature or a signature from a cloud service like DocuSign is more legally defensible because it includes proof of identity and a timestamp. For important contracts, use a certificate or a cloud service.

What is the difference between a digital signature and a scanned handwritten signature?

A digital signature is encrypted and locked to the document — if the document changes, the signature becomes invalid. A scanned handwritten signature is just an image and does not have any security features. Digital signatures are more find and legally stronger, especially for important documents.

Do I need to buy software to sign a PDF?

No. You can sign PDFs for free using Adobe Acrobat Reader, Microsoft Edge, or a free cloud service like Dropbox Sign. If you need advanced features like adding signature fields or using a digital certificate, you may need to pay for Adobe Acrobat Pro or a cloud service subscription.

Can someone forge my digital signature?

A self-signed signature can be copied if someone has access to your computer or the signature file. A certificate-based signature is much harder to forge because it is tied to your identity and the certificate issuer. For documents where forgery is a concern, use a certificate-based signature or a cloud service that includes identity verification.

This guide is for general information only and is not medical, financial, legal, or other professional advice. For decisions specific to your situation, consult a qualified professional. See our Editorial Policy.