True if the signature and document look well formed and well
A signature with a PDF document occupies a specific portion of
that document and should cover the rest of the document. However it
is possible to create signatures which conform technically but are
badly or suspiciously applied.
For example one could create a signature which did not cover the
start of a document. While this is legal it is not sensible and
this kind of structure should be regarded as insecure.
This property can be used to determine whether the signature has
been well and sensibly applied.