If I got you right, you search for documents using CSOM/REST.
Let's assume you try to use 'query' method of '_api/Search' endpoint.
All you need is to define selectproperties for query:
You may need SPWebUrl to construct request to REST endpoint for item properties.
Let me assume you're using on-premises SharePoint Server 2013 installation.
In this case you have to create new text managed property in Search Service Application's schema (don't forget to mark it as Retrievable):
Also don't forget to do full crawl after property has been added.
SharePoint Online already has it mapped to "SPWebUrl, owstaxIdSPLocationSite" (not enough reputation to post another link :) )
Hope this help.