Multiple digital signature validation troubles

[Shurmano]

Hi all,

we received PDF documents from arquitects and this kind of professionals. This kind of documents have 2 group of signatures: 

- Arquitect's personal ones

- Official Visa Signature of the Official College of Architects: this signature stamps an image on all pages.

So we have this message:

If we get the signed version of the 3 first signatures it's ok.

Here in Spain, the "official" validator (https://valide.redsara.es/valide/validarFirma/ejecutar.html) it's based on "ETSI TS 102 778-1 V1.1.1 (2009-07)" ("Electronic Signatures and Infrastructures (ESI); PDF Advanced Electronic Signature Profiles; Part 1: PAdES Overview - a framework document for PAdES") specification.

So it says that, individually, the signature are correct. In fact, the bytes of the first 3 signatures, are not modified by the last ones (although that added image is not into a Form)

BUT, really, that image has a clear visual impact on the final PDF. But I have no technical arguments to reject that kind of PDFs.

Senders say that if the signature are correct according that validation and specification, we have to accept it.

¿What is the exact Adobe Reader's digital signature validation?

Thanks in advance