This post is about aspects to take into account regarding PDF format and the security.
PDF files are considered “immutable” and are used by many people to handle sensible documents, such as contracts or agreements. However, these documents are not immutable and can be altered.
Checking PDF Validity
Check that PDF has not been altered with software (e.g. Neevia).
Check that PDF has been signed, and that the signature is valid.
If it uses Acroform or XFA, it may contains scripts that could be used with malicious intentions.