PDF Signatures violate the The Cryptographic Signature Doom Principle, which is that your signing scheme will lead to doom unless the signature wraps the entire

PDF Signatures violate the The Cryptographic Signature Doom Principle, which is that your signing scheme will lead to doom unless the signature wraps the entirety of your document's binary representation. Same for XML signatures. Violating the TCSDP doesn't mean that your scheme is technically wrong, but it does mean that implementations will often be wrong in funny ways.