Hi, thanks for your fast reply.
From the url http://www.telecomitalia.com/tit/en/innovazione/digital-champions/fatturazione-elettronica.html :
The electronic invoice is technically an electronic XML file(eXtensible Markup Language).
In order to be accepted, the invoice must have the following characteristics:
it must be unchangeable and unalterable (implemented by means of the inclusion of a digital signature of the issuer)
it must state the date of issue
it must be authentic with intact contents
The law also establishes that in order to simplify matters for businesses, accredited intermediaries can be used to send and store the invoices.
also:
The Exchange System distinguishes between three types of file:
FatturaPA file: a digitally signed XML that complies with the FatturaPA format specifications. It may contain:
a single invoice (only one invoice body)
an invoice lot (several invoice bodies with the same heading).
For the FatturaPA file signature, see the Sign the FatturaPA section.
archive file: compressed file (exclusively in zip format) containing one or more FatturaPA files.
The system processes the archive, checking the individual FatturaPA files contained within it and forwarding it to the addressee. In practice the FatturaPA files are treated as if they were transmitted individually.
The archive file must not be signed but, instead, all the FatturaPA files in it must be signed.
message file: an XML file compliant with a schema (xml schema) described by the file MessaggiTypes_v1.1.xsd which can be downloaded in the Exchange System documentation section.
A message file can be:
a rejection notice: a message sent by the ES to the transmitter whenever the file transmitted (FatturaPA file or archive file) fails the checks provided for;
a metadata file: a file that the ES sends to the addressee, together with the FatturaPA file;
a delivery receipt: a message sent by the ES to the transmitter to certify that the FatturaPA file has been delivered to the addressee;
a failed delivery notice: the message sent by the ES to the transmitter to report that the FatturaPA file could not be delivered to the addressee;
a client outcome notice: a message that the addressee sends to the ES to report the acceptance or rejection of the invoice received; the report can be received by the ES within a deadline of 15 days;
a outcome notice: a message sent by the ES to the transmitter containing the decision reported by the addressee in the client outcome notice;
a client outcome rejection notice: a message sent by the ES to the addressee to report a situation where the client outcome notice is not admissible or non-conforming;
a deadline passed notice: a message sent by the ES to both the transmitter and the addressee, when 15 days have passed and no client outcome notice has been received, informing both parties that the period within which the system accepts the outcome notice message has passed;
a file undeliverable notice: a message sent by the ES to the transmitter when, after 15 days from the date of the failed delivery notice, it has not been possible to deliver in any way the FatturaPA file to the addressee.
To view some examples of message files go to the Exchange System documentation section
The Exchange System gives a different name to each message file type. For more information on naming message files see Annex B-1 of the document Technical specifications of the Exchange System - draft - pdf in the Exchange System documentation section.
All messages produced and sent by the Exchange System, with the exception of the metadata file, are signed electronically with an advanced, XAdES-Bes electronic signature.
The client outcome notice, the only notice sent by the addressee to the ES, may optionally be signed electronically, again in XAdES-Bes mode. For more information on the client outcome notice see the Communicating the outcome for the FatturaPA section.
It's not a simply pdf format. But... complex .xml file!
I attach a screenshot from a SimplyFatt2, an OS X software that i use for my job. This screenshot show how can software create electronic billing.