There are two kinds of signature you might mean:
- An electronic signature: your signature drawn, typed or pasted onto the page. This is what most forms and everyday agreements need.
- A digital signature: a cryptographic seal made with a certificate, which proves who signed and that the document hasn’t changed since. Some contracts, government forms and regulated industries require this.
On a Mac: Preview
Preview, which comes with every Mac, can add a drawn signature. Open the PDF, click the Markup button, then Sign, and create a signature with your trackpad or your camera. Drag it onto the page and save.
Preview doesn’t do digital signatures with certificates.
On Windows: Microsoft Edge
Edge can draw on a PDF, which works as a quick signature with a mouse, pen or touchscreen. It doesn’t have a proper signature tool or certificate support.
On Windows or Mac: Paperwing
Paperwing is a free, open-source desktop app that does both kinds of signature, offline and with no account.
To add your signature:
- Download Paperwing and open your PDF (drag it onto the window, or Ctrl+O / ⌘O).
- Open All tools and choose Fill & Sign.
- Draw your signature, type it, or upload a picture of it.
- Place it on the page and resize it. Fill in any other fields while you’re there.
- Save with Ctrl+S (⌘S on a Mac).
To add a digital signature: choose Digital Signature in All tools, select your X.509 certificate file and sign. You can add an RFC 3161 timestamp from a timestamp server you choose, which records when the document was signed. Only a hash of the document is sent to that server, never the document itself. To check signatures on a PDF someone sent you, use Validate Signature.
Is an electronic signature legally valid?
In many countries, including the US (ESIGN Act) and the EU (eIDAS), electronic signatures are valid for most agreements. Some documents, like certain property, court or notarized papers, have stricter rules. If in doubt, ask the person who sent you the document what they accept.
Why sign on your computer?
E-signature websites ask you to upload the document and usually to create an account. Signing on your computer keeps the contract between you and the person you send it to.
Quick answers
Can I sign a PDF for free? Yes, with Preview on a Mac, or Paperwing on Windows and Mac.
Can I sign on a phone? Paperwing is a desktop app for Windows and macOS. On a phone, use the markup tools your phone comes with.
How do I stop someone changing the document after I sign? Use a digital signature, which shows if the file changes, or flatten the PDF so the fields can’t be edited.