Opening files not in current project

Sorry if this is documented somewhere - I've looked but am probably searching for the wrong terms.

Recently the way files outside of the current project changed to opening in a separate instance of phpstorm - more like a web page.  My question is how do I open those same files within the current project instead of a separate windows?

The only files I typically open are project related anyway but are more often shared files in a common shared directory on the server.

Thanks in advance.