How to sign a PDF

▶ 1. Image signature and digital signature are not the same thing

This is the most important distinction in this guide, because the two signatures answer different needs and carry different weight.

• ✓ Image signature — a picture of your signature laid on the page. Fast and legible, but it is only a drawing: anyone could copy it from another document.
• ✓ Digital signature — a cryptographic operation based on a certificate. It ties the document to a verified identity and makes any change made after signing evident.

For an internal form, a receipt or an informal letter, an image signature is enough. For a contract, a filing with a public authority or any document that has to stand up to a challenge, a digital signature is required.

✎ 2. Adding an image signature

The signature is uploaded once and stays available for later use, stored locally in the browser.

1. Sign on a blank sheet, photograph or scan the signature and crop it.
2. Install the extension from the download page and open the document with Choose a PDF.
3. Open Add signature and upload the file: PNG and JPG are accepted.
4. Select the signature and click Place selected signature.
5. Position it on the page and adjust its size by dragging the handles.
6. Click Download PDF to save the signed document.

🔒 3. Applying a digital signature

Here you need a personal certificate, that is a file attesting your identity and protected by a password.

1. Open Digital signature.
2. Upload your certificate in .p12 or .pfx format and enter the certificate password.
3. Choose the signature profile: PAdES-B, the standard used across Europe, or Adobe PKCS#7.
4. Fill in the details that accompany the signature: displayed signer name, reason, location and contact information.
5. If you want the signature to be visible on the page as well, switch on visible signature on page and position it.
6. Click Digitally sign and download.

🏆 4. How a certificate is obtained

A certificate is not something you create yourself: it is issued by a recognised body after verifying your identity.

1. Signing certificates are issued by qualified trust service providers, accredited by the relevant national authority.
2. Identification takes place in person, through a national digital identity scheme, or by video call, depending on the provider.
3. At the end you receive a certificate that can be exported in .p12 or .pfx format, or that lives on a physical device such as a smart card or a USB token.
4. To use it here you need the .p12/.pfx file: if your signature is tied to a hardware device, you will have to use the provider's own software.

✓ 5. Verifying a signature

After signing it is worth checking that everything is in order before sending the document out.

1. Open the signed PDF with a full reader, which shows a dedicated panel with the status of the signature.
2. Check that the signer matches and that the document is reported as intact, that is unchanged since signing.
3. Remember that any later change — even just adding a note — invalidates the signature: that is why signing comes last.
4. If you still have something to change, go back to the unsigned document and sign again at the end.

◆ 6. After signing

A signed document often contains personal data: a few extra precautions before sharing it make sense.

1. With Password protection you can encrypt the document in AES-256 and limit in Allowed actions what recipients may do.
2. Bear in mind that touching the file after a digital signature compromises its validity: if you need to protect it, consider doing so before signing.
3. For a form to be signed by others, see the guide on how to create a PDF form.

? 7. Frequently asked questions

The questions that come up most often on this topic.