Validly signed PDF form without harmful behavior

A valid PDF signature is not a general trust signal, but it is useful context for low-risk fillable forms. This tier fires only when the signature verifies, there is no invalid or post-sign active-content change, no exploit/CVE/AV/ML hit, no launch action, no embedded payload, no suspicious external URL, and any JavaScript is limited to Acrobat form-helper calls such as date and number validation.