The Keywords standard PDF metadata field is mapped, by the
PDF-ShelTools Property Handler, to the Windows Tags (System.Keywords) property, and the Property Handler is also used by the Windows Search Indexer to index all the properties it exposes, so yes, you just need to edit that Tags property, using the Shell Details Pane or file properties details tabsheet, to edit and have that metadata property indexed.
If it is not working for you, try
rebuilding the Windows Search index
