The official application is AutoFirma
AutoFirma is the electronic signature application developed by the Spanish administration. It is free to download from the Government's electronic signature portal, works on Windows, macOS and Linux, and can run from the browser when an electronic office asks you to sign inside a procedure. Its aim is that you do not have to choose a signature format: the application picks the right one.
The same portal publishes installation manuals and user guides, including how to fix the most common installation failures.
What that signature is worth
It is not an image of your signature pasted into the PDF. It is a cryptographic signature tied to your certificate, proving who signed and guaranteeing that the document has not been changed since. Law 6/2020 gives full legal effect to the electronic signature systems provided for in administrative rules.
Signing is not the same as scanning. A PDF with a scanned handwritten signature does not have the value of an electronic signature: it does not prove who signed it or stop the document being altered afterwards.
Common problems
- AutoFirma sees no certificate. The certificate is not in the system store, or it has expired. Review how to install it.
- The office asks you to sign and the application does not start. Usually AutoFirma is not installed or the browser blocks it. Reinstall it from the official portal and restart the browser.
- The recipient says the signature is not valid. Check you are sending the signed PDF and not the original, and that it was not saved again or compressed after signing: any later change invalidates the signature.
An electronic signature is not a scanned signature
Pasting an image of your handwritten signature into a PDF is not an electronic signature: it does not prove who signed, does not guarantee the document was not changed afterwards and lacks the legal effect the administration requires. It is a drawing.
A certificate signature does all three: it identifies the signer, binds the signature to the specific content of the document and detects any later alteration. That is why registries, courts and electronic offices accept the second and reject the first.
How to check a signature is valid
A signed PDF can be verified without taking anyone's word: the official VALIDe service checks the certificate, its validity and the integrity of the document, and returns a report. It is the tool we use when a client sends us a «signed» document and we need to know whether it is usable.
If the report says the certificate was expired or revoked when signing, the signature is not valid, even if the PDF opens without warnings in the reader.
Common problems and their cause
AutoFirma does not start or does not see the certificate. It is almost always one of three things: the application is out of date, the runtime it needs is missing, or the certificate is in Firefox's store and AutoFirma looks at the system one. Installing it in both stores solves most cases: see installing the certificate.
The PDF comes out «signed» but the office rejects it. Usually the office requires a specific signature format, PAdES in most cases, and another was produced. AutoFirma lets you choose the format in its settings.
You sign several times and the previous one disappears. Use cascading signatures, do not sign over the original document again. Each added signature must respect the earlier ones.
The signed document is much larger. That is normal: the signature embeds the certificate and, depending on settings, a timestamp. A PDF that grows by a few hundred kilobytes when signed is fine.
And a precaution that saves trouble: always sign the final version. Any later edit, even adding a note, breaks the signature.
A document with a deadline and a signature that will not work?
We sign it and file it with the relevant body using the professional certificate of a registered gestor.
Frequently asked questions
Is AutoFirma free?
Yes. It is an application of the Spanish administration and downloads free from the official electronic signature portal.
Can I sign a PDF without a digital certificate?
Not with a certificate signature, though some procedures accept other systems. For a signature with full effect the certificate is the norm.
Does a finger signature made on a phone count?
Not as an electronic signature. It is a drawing: it proves neither identity nor the integrity of the document.
Can a PDF already signed by someone else be signed?
Yes, signatures can be chained. Each signer adds theirs without invalidating the earlier ones.
Legal basis
- Law 6/2020 regulating certain aspects of electronic trust services
- Law 39/2015 on Common Administrative Procedure, art. 14 on electronic dealings with the administration
- Royal Decree 203/2021, Regulation on public sector action by electronic means
Official sources
- FNMT: individual certificate
- Spanish Government electronic signature portal: downloads
- Importass, portal of the Social Security Treasury
- Social Security electronic office
Written by Irina Klein. Reviewed by Elena Alonso Volostnova, registered gestora administrativa (ICGA Valencia). Updated on 5 October 2026. This page is general information and does not replace personalised advice on your case.