You're talking about files that are already in Zotero, and you want to be able to do a search externally but open them as if you'd double-clicked on them within Zotero?

That's not possible directly, and I think it would be fairly weird behavior. We'll soon be making it possible to add PDFs to Zotero by opening them via the app, but I think overloading that with a second action would be messy and confusing. If you're in any groups, it will need to prompt for what library to add the PDF to, but then you have some convoluted dialog that forces you to choose between 1) adding the PDF to a particular library or 2) opening the PDF in one or more existing libraries where it might exist, based on matching the local file path against paths in the database.

The proper solution is opening Zotero items directly outside of Zotero, and there's an Alfred plugin that might be able to do that. (If not, the developer could easily add support for launching zotero://select or zotero://open-pdf links.) We don't currently have any plans for a Spotlight plugin for Zotero items, but that would also be an appropriate option for this.