[SOLVED]
I get now the same PDF Invoice with my custom field when generated by the Plugin (Attached invoice) or by the Administrator on the Order List Page.
I have founded a second address_template file in Display->Views main menu, then:
my custom field tag have to be in ADDRESS View address_template, but also in ORDER View address_template.
I don't know if that is the right way, but now it works correctly.
Thanks al lot.
Now I have to solve a bug with a 100% discount Coupon generated by HikaSerial...