How to Sign PDF Documents Online in Portugal With the Chave Móvel Digital

Many things in Portugal can be done without printing anything, and signing a document is one of them. If you have a Chave Móvel Digital (CMD), you can sign PDF files from your phone, free of charge, and the signature can be checked by anyone. Here is how it works.

What the Chave Móvel Digital is

The CMD is the Portuguese state's digital identity key. It is built into the gov.pt smartphone app, which replaces the former Autenticação.gov app. Besides signing documents, it is the key you use to log in to many public services online. For example, it is needed to request the certificate of tacit approval when AIMA does not answer a renewal, which we explain in our guide on AIMA delays.

Why sign with it

• It is free.
• You do not need to print, sign by hand, scan and send.
• The signature is tied to your registered identity, so the other side can verify who signed.

How to sign a PDF

1. Save the PDF on your phone, in the Files app on iPhone or the file browser on Android.
2. Open the gov.pt app. In the quick access list, choose Assinar documentos (sign documents).
3. Select Assinatura própria (your own signature).
4. Pick the PDF from your phone.
5. Follow the login steps, then drag the signature box to where you want it to appear on the page.
6. Export the signed file with the share button, and save it or send it by email or any other app.

What the visible signature box really is

The little signature on the page is only a visual marker. What makes the signature real is invisible: the process adds cryptographic data to the file that can be verified, and that also reveals if the document was changed afterwards.

How to check a signed document

If someone sends you a document signed this way, you can verify it yourself on the official Portuguese signature validation portal (Validador de Assinaturas e Certificados):

1. Open the validator and go to the section to validate signatures.
2. Upload the signed PDF (or ZIP) using the signed file option.
3. Click Validar.

You get a simple report with the name of the person who signed, the exact time it was signed, and whether the document was altered after signing.

Practical tips

• Sign only the final version. Any change after signing breaks the signature.
• Keep the signed PDF, not a screenshot or a print.
• Ask the other party whether they accept electronic signatures before you rely on one, especially for contracts with banks or landlords.

If you need help with documents for your visa, bank account or company, we can tell you which ones can be signed digitally and which still need an original.

General information. Check the requirements of each recipient before you rely on a digital signature.