How to Sign a PDF

Signing a PDF can mean two very different things: adding a cryptographic digital signature that proves who signed it and that the document has not been changed since, or simply placing an image of your handwritten signature on the page. The right choice depends on what legal weight, what counterparty, and what tooling you have. This guide covers both, with trusted tools for each.

Digital signature vs electronic signature

The terms are often used interchangeably, but they are not the same thing.

• Digital signature. Cryptographic. The signer holds a private key (usually backed by a certificate from a trusted authority) and the signature is a value computed over the document's bytes. A reader can verify the signature with the signer's public key and detect any change to the document after signing. This provides integrity, authenticity, andnon-repudiation (within the limits of the PKI).
• Electronic signature. Broader. Any electronic means of indicating intent to sign — a typed name, a click on an "I agree" button, or an image of a handwritten signature pasted onto the page. E-sign platforms (DocuSign, Lumin, Adobe Sign) wrap this in an audit trail, but the signature itself is not necessarily cryptographic.

In many jurisdictions (ESIGN in the US, eIDAS in the EU) a basic electronic signature is legally binding for many kinds of contracts. For high-stakes documents — notarized forms, regulated filings — a qualified digital signature with a certificate from a recognized authority is often required.

Tools for signing a PDF

1. Preview (macOS, free) — electronic signature

Open the PDF in Preview, click the Sign button in the toolbar, and create a signature using your trackpad, camera, or iPhone. Drag the signature onto the page and save. Preview stores the signature image so you can reuse it. This is an electronic signature — fast, free, and sufficient for most personal and small-business use.

2. Adobe Acrobat Reader (free) — digital or electronic

The free Reader can apply both kinds of signature. For an electronic signature, use Fill & Sign and draw or type your signature. For a digital signature, useCertificates (under Tools) — you will need a digital ID, which you can create a self-signed one for free or obtain from a trusted authority for a fee. Reader shows a signature panel that reports whether the signature is valid and the document has not been modified since signing.

3. DocuSign (paid, cloud) — electronic with audit trail

DocuSign is the market-leading e-sign platform. You upload the PDF, place signature fields, and the counterparty signs via email. The platform records a detailed audit trail (who, when, IP, hash). The trade-off: the document passes through DocuSign's servers. For documents where a third-party audit trail matters more than local processing, this is the standard choice.

4. Lumin (freemium, cloud) — electronic, PDF-focused

Lumin is a PDF-focused e-sign tool with a free tier for basic signing. Similar workflow to DocuSign — upload, place fields, sign. Useful when you need a quick e-signature and the document is not sensitive enough to require local processing.

Step-by-step: sign a PDF in Preview (macOS)

1. Open the PDF in Preview.
2. Click the Sign icon in the toolbar.
3. Create a signature: Trackpad (draw with your finger), Camera (sign on paper and hold it up), or iPhone (draw on the phone screen).
4. Drag the signature to the right place on the page; resize if needed.
5. Save the PDF. The signature is now part of the page.

Step-by-step: apply a digital signature in Adobe Reader (free)

1. Open the PDF in Adobe Acrobat Reader.
2. Go to Tools > Certificates.
3. Click Digitally Sign and drag a rectangle where the signature should appear.
4. If you have no digital ID, Reader will walk you through creating a self-signed one (or you can load a certificate from a file or a security provider).
5. Save the PDF. The signature panel will show "Signature is valid" if the document has not been modified since.

Verifying a signature

For digital signatures, open the signed PDF in Adobe Reader and look at the signature panel (left side). It reports whether the signature is valid, the signer's identity, the time of signing, and whether the document was modified after signing. For electronic signatures, there is nothing to verify cryptographically — the signature is just a mark on the page; trust comes from context and, for platform signatures, the audit trail.

Common mistakes

• Confusing the two kinds of signature. A pasted image is not a digital signature and provides no tamper-detection.
• Using a self-signed certificate for something that needs a trusted identity. Self-signed is fine for internal use; it proves nothing to a third party.
• Modifying the document after signing. Any change after a digital signature is applied invalidates it. Sign last.
• Uploading a sensitive contract to a random e-sign site. Use a reputable platform or, for the most sensitive documents, a local tool.

Related IXPDF tools you can use locally

While IXPDF does not yet sign PDFs, several existing tools are useful in a signing workflow:

• Rotate PDF — fix page orientation before signing.
• Edit Metadata — set the document title, author, and subject before sending for signature.
• Merge PDF — combine the document and a signature page into one file.
• Compress PDF — shrink the signed file for email.