How to sign a PDF with a digital certificate on iPhone or iPad

What you need

• The certificate with its private key, as a .p12 or .pfx file. A .cer or .crt file is only the public part and can’t sign.
• The file’s password, set when the certificate was exported.
• An app that signs with certificates on iPhone or iPad. Not every PDF app does: several popular ones only offer drawn signatures on mobile.

If your certificate lives on a computer, export it from there with the private key included: in a browser’s certificate settings, in the Windows certificate manager, or in Keychain Access on a Mac (choose the certificate together with its key and export as .p12). Send the file to your iPhone by AirDrop or save it in Files. Treat that file like a key to your identity: don’t email it to anyone and delete stray copies.

Why installing it in Settings isn’t enough

When you open a .p12 on an iPhone, iOS offers to install it as a profile. That puts the certificate in the system’s keychain, where Apple’s own apps such as Safari and Mail can use it, for example to log in to a government website. Other apps generally can’t reach it there. That’s why a certificate can “work in Safari” and still not appear in a PDF app. The app needs its own copy, imported inside the app.

Three ways that work

1. Your issuer’s own app

Some certificate authorities and governments publish an app that signs PDFs with their certificates. In Spain, for example, the FNMT’s official app added PDF signing on iOS and Android in 2024. These apps are free and know their certificates well, though they’re usually limited to signing rather than filling or editing the document.

2. A PDF app that imports your certificate

Apps that support certificate signatures let you import the .p12 once and sign any PDF. Look for “PAdES” in their description, which is the European standard format for PDF signatures and the one most validators expect.

3. A cloud signing service

Some issuers keep your key on their server and sign remotely. That avoids handling the file, but your document, or at least its fingerprint, goes through their service.

Step by step

1. Import the .p12 into the app and enter its password.
2. Open the PDF and choose an empty signature field, or draw the area where the signature should appear.
3. Choose whether the signature shows your name and the date on the page. The visible part is decoration; the real signature is inside the file.
4. Sign, and confirm with Face ID, Touch ID or your passcode if the app asks.
5. Save once. Don’t compress, print or convert the file afterwards: that would invalidate the signature.
6. Check it: open the signed PDF in a validator and confirm it shows as valid and signed by you.

Will the recipient accept it?

That depends on them and on the law that applies. In the European Union, the eIDAS Regulation defines electronic, advanced and qualified electronic signatures; a qualified signature made with a qualified certificate on a qualified device has the same legal effect as a handwritten one. A certificate in a .p12 file can produce an advanced signature, which many procedures accept. If a procedure demands a particular level or tool, follow what it says.

If the recipient sees “validity unknown”, the signature is usually fine and their software simply doesn’t trust your certificate’s issuer yet. Here’s how to tell the cases apart.