Per abilitare la convalida a lungo termine, occorre:
dopo aver firmato il PDF, salvarlo con il comando "Salva come altro..." / "PDF con funzionalità avanzate di Adobe Reader" / "Abilita creazione commenti e misurazione..."
No, this does not enable long-term validation.
If you already have a signed PDF you need Acrobat 9.1 or later to do that. Right-click on a signature and select "Add Verification Information". Save PDF. The long-term validation will be honored in Acrobat/Reader 9.1 and later. The signature must be Valid in order for the "Add Verification Information" command to work.
If you are just signing a PDF do the following before the signing: Go to Edit->Preferences->Security->Advanced and select "Creation" tab. Check "Include signature's revocation status when signing" check box. This works in Acrobat 8 and earlier versions. In Acrobat 9 and later this is the default. In versions earlier than 9 the default was that this check box was unchecked. In Acrobat XI the location of this dialog has moved. It is now in Edit->Preferences->Signatures->Creation&Appearances->More...
If you have PDFs that have many signatures it is better to uncheck this check box and to use "Add Verification Information" command, provided that the recipients of these PDFs have newer Reader/Acrobat versions. This makes PDFs with multiple signatures smaller.