What a digital signature does and why it matters

A digital signature is a way to sign a PDF document electronically so that the recipient knows it came from you and hasn't been changed since you signed it. Unlike a typed name or a scanned image of your handwriting, a real digital signature uses encryption — a mathematical lock that proves you signed the document and proves the document hasn't been altered after you signed it.

Most PDFs you'll encounter don't need a digital signature. A contract you email back with your name typed at the bottom works fine for most purposes. But if you're signing something legally important — a mortgage, a power of attorney, a court filing — a digital signature creates a record that holds up in court and can't be forged or edited without breaking the signature.

The process differs depending on whether you're using Adobe Acrobat (the paid version), free tools like Preview on Mac, or online services. All of them work, but they create different levels of legal weight.

Key Takeaways

  • A digital signature uses encryption to prove you signed the document and that it hasn't been changed since you signed it, which is different from a typed name or scanned handwriting.
  • Adobe Acrobat Pro (paid) creates the strongest digital signatures and works with most legal and financial documents.
  • Free tools like Preview on Mac or web-based signers like DocuSign create signatures that work for most everyday documents but may not carry the same legal weight as Adobe signatures.
  • You need either a certificate from your computer's operating system or a third-party service to create a real digital signature; you cannot create one from scratch without one of these.
  • After you sign, the document is locked so that any changes made after signing will break the signature and show as invalid.

Using Adobe Acrobat Pro to sign a PDF

Adobe Acrobat Pro is the most widely recognized tool for digital signatures. Open your PDF in Acrobat Pro, click the Sign button in the right panel, then select Add Signature. You'll be asked whether you want to sign yourself or request a signature from someone else — choose to sign yourself.

The first time you sign, Adobe will ask you to create a signature identity. You can draw your signature with your mouse or trackpad, type your name, or upload an image of your handwritten signature. Adobe stores this as your default signature. After that, every time you sign a document, you just click where you want the signature to appear and confirm.

Adobe signatures are tied to your Adobe account and use encryption that most legal and financial institutions recognize. If you're signing something that will be reviewed by a bank, a court, or a government agency, Adobe Acrobat Pro is the safest choice.

Creating a signature on Mac using Preview

If you have a Mac, Preview (the built-in image viewer) can add a signature to a PDF without any extra software. Open the PDF in Preview, click the Markup Toolbar button (it looks like a toolbox), then click Signature. You'll see options to sign with your trackpad, your camera, or a saved image of your signature.

Preview signatures are simpler than Adobe's — they're essentially an image placed on top of the PDF rather than an encrypted digital signature. This works fine for everyday documents like permission slips, rental agreements between friends, or internal company forms. For anything legally binding or that will be reviewed by a financial institution, use Adobe Acrobat Pro instead.

After you add your signature in Preview, save the file. The signature becomes part of the PDF and won't change if someone opens it later.

Using online signature services like DocuSign or HelloSign

Online services like DocuSign, HelloSign (now Dropbox Sign), and Adobe Sign let you upload a PDF, add your signature, and send it to others for their signatures too. These services are designed for documents that move between multiple people — contracts, agreements, forms that need approval chains.

To use one of these services, upload your PDF to the platform, click where you want to sign, draw or type your signature, and confirm. The service creates a record of when you signed and from what device, which adds a layer of proof beyond the signature itself. Many of these services offer a free tier that covers a few documents per month, though they charge for higher volume or advanced features.

These signatures are legally recognized in most U.S. states under the ESIGN Act, which means they hold up in court for most contracts. However, some documents — like wills or powers of attorney — may have state-specific rules about what kind of signature they require, so check your state's laws if you're signing something unusual.

What you need before you can sign digitally

To create a real digital signature (not just an image of your name), you need either a certificate or an account with a signing service. A certificate is a digital file that proves who you are. Your computer may already have one — Windows and Mac both create basic certificates automatically — but for legal documents, you often need a certificate from a trusted third party.

If you're using Adobe Acrobat Pro, DocuSign, or HelloSign, you don't need to worry about certificates yourself — the service handles it. If you're using Preview on Mac or a free PDF tool, you're creating a signature image rather than a cryptographically signed document, which is fine for most everyday use but not for legally binding contracts.

For high-stakes documents like mortgages or court filings, ask the institution receiving the document what kind of signature they require. They'll tell you whether Adobe, DocuSign, or a specific certificate provider is acceptable.

How to sign a PDF on Windows without paid software

Windows doesn't have a built-in PDF editor like Preview on Mac, so your free options are more limited. You can use Microsoft Edge to open a PDF, click the Draw button, and draw your signature directly on the document, then save it. This creates a signature image, not an encrypted digital signature, but it works for documents that just need proof you've reviewed them.

For a stronger signature on Windows, use a free online service like Canva, Smallpdf, or ILovePDF. Upload your PDF, add your signature (drawn, typed, or uploaded), and read the signed version. These tools create signature images rather than encrypted signatures, so they're best for everyday documents rather than legal contracts.

If you need a real encrypted digital signature on Windows and don't want to pay for Adobe, your best option is a free account with DocuSign or HelloSign, which both offer limited free signing per month.

What happens after you sign and how to verify the signature

Once you sign a PDF, the document is locked at that point in time. If anyone tries to change the document after you've signed it, the signature will show as invalid or broken. This is the security feature that makes digital signatures useful — it proves the document hasn't been tampered with since you signed it.

To check whether a signature is valid, open the signed PDF in Adobe Acrobat or the service that created it. Look for a signature panel or badge that shows the signer's name, the date and time of signing, and whether the signature is valid. If the document has been edited since signing, the signature will display a warning.

If you're sending a signed document to someone else, they can verify your signature the same way. If you used Adobe Acrobat Pro or a service like DocuSign, they'll see your name and the timestamp. If you used Preview or drew a signature in Edge, they'll see the image but won't have the encryption proof that it came from you.

Frequently Asked Questions

Can someone forge my digital signature?

A real encrypted digital signature (from Adobe Acrobat Pro or DocuSign) cannot be forged because it's tied to your account or certificate. An image-based signature (from Preview or Edge) can be copied and pasted, so it's less find. For important documents, use Adobe or a signing service.

Do I need to print and scan a document to sign it?

No. You can sign any PDF directly on your computer using the tools described here. Printing and scanning is slower and creates a lower-quality image, so use digital signing instead.

What if the document asks for a "wet signature"?

A wet signature means a handwritten signature on paper. Some legal documents, especially wills and powers of attorney, may require this. If the document specifically says wet signature, you'll need to print it, sign it by hand, and scan it back in. Check with the institution sending the document to confirm.

Can I sign a PDF on my phone?

Yes. DocuSign, HelloSign, and Adobe Sign all have mobile apps that let you sign PDFs on your phone. You can also use Preview on an iPad to sign PDFs. The process is the same as on a computer — draw or type your signature where indicated and confirm.

Is a digital signature the same as a signature on a digital document?

No. A digital signature is an encrypted proof that you signed the document. A signature on a digital document could be just an image of your name or handwriting. For legal protection, you want a real digital signature from Adobe, DocuSign, or a similar service.