On-premises SharePoint 2013 Enterprise Search Farm: for about 99.99% pdf documents (millions in the document library) search by Managed Property, CustomDocumentId (unique per doc), returns result. However, for a few, the similar query doesn't return results. Crawled history shows that document was crawled. Also, search by a different managed property like ProviderNumber finds the document. The property shows that it has a value. We can rule out the security as running queries as the Site Collection Administrator.
This works for most of the documents, but not for a few
This returns the documents along with other documents