Search MS Office files by custom properties

on a W2008R2 server with File Server role we are archiving 2013 Office files, for which the users are using custom properties (as of this link), eg.:

• "doctype": "notification"
• "vectortype": "priority mail"

I am trying to search for those files by their custom properties, but I am getting no results: I am following Microsoft rules for searching files (see this link), eg.:

• doctype:notification
• vectortype:priority

Could you please give me any hint? Thanks in advance