The revision of the document in which this object is stored.
PDF documents can be incrementally updated so that changes are
appended to the document rather than overwriting the original. This
means that it is possible to revert back to a previous version of
the document. This feature is mostly used for document signatures
so that the act of signing the document does not invalidate
Objects from the oldest update will have a Revision of one and
and larger values indicate more recent updates. A value of zero
indicates that the object was not read from stream.