It is possible to create smart rules and groups using PDF Annotations as a criterion and also to do advanced search based on the value of PDF Annotations. I would like to include this metadatum in custom metadata in the inspector, but it doesn’t show up in the drop down list and adding a field with that name doesn’t work. Is that possible?
No annotations can’t be used in this way.
PDF Annotations are not really “metadata”, as they are part of PDF pages (checkout Apple’s PDFKit documentation). And they are not easily searchable, as they can be a lot of other things beside text – how would you search for a red ellipse drawn in a 3 pixel wide dotted line?
You could copy PDF Annotations to custom metadata fields with a script. But in my mind, that is not sensible: An Annotation can run over lines and lines of text or be just a highlight. Mapping that to custom metadata is tedious.