...

If I were you I would use the same term provided by the multilingual website of the same Ministerio de Justicia that issues the certificate. Please, visit this web page (choose the English version): https://sede.mjusticia.gob.es/cs/Satellite/Sede/en/servicios...

There it says:
The electronic document with its original signature can be retrieved using the *secure verification code (SVC)* shown on the printable version of the document.
You can do this in the following procedures:
[…]
The Ministerial Order establishing the *Secure Verification Code* system can be viewed in Regulations.


This *Secure Verification Code* is the one set forth by the Ley 11/2007, de 22 de junio, de acceso electrónico de los ciudadanos a los Servicios Públicos, available on line at:
http://noticias.juridicas.com/base_datos/Admin/l11-2007.t2.h...

You can also google: “Orden JUS de 10 de enero de 2011 por la que se establece el sistema de códigos seguros de verificación de documentos en el desarrollo de actuaciones automatizadas del Ministerio de Justicia”.

With respect to *PN*, this means *penales*. As the SVC can be used in several procedures, here the *PN* is indicating that it was used for asking a *criminal record certificate*. You can translate it as *CRIM*, for example.

Good work!


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Note added at 19 hrs (2016-05-30 10:26:45 GMT)
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Yes, fully I agree with you and with Amelia; you should maintain *PN* and include a footnote explaining its meaning.