@Override
        public String toString() {
            return "MCID{" + "objectRef=" + objectRef + ", mcid=" + mcid + '}';
        }
In PDF land, MCID are integers that should be unique _per page_.
This class includes the object ref to the page and the mcid
so that this should be a cross-document unique key to
given content.
<p>
If the mcid integer == -1, that means that there is text on the page
not assigned to any marked content.
tika-parsers/tika-parsers-standard/tika-parsers-standard-modules/tika-parser-pdf-module/src/main/java/org/apache/tika/parser/pdf/PDFMarkedContent2XHTML.java