Change default app for all files in a folder

There can be only one 'default' for the whole system. However, you can of course set one or more files to associate with another app from the default.

In the Finder, you can select multiple files of the same type within a folder, then press Command Option I, which brings up one "Get Info" dialog for all the files.

(In fact, they don't even have to be the same file type, as long as one app can open all of the types.) You can then change the app that opens those files on a double-click.

Obviously, any files added subsequently would need to be changed manually after the fact.

You could create a Folder Action to modify the opening app for any file dropped into a given folder.

This is not how it works.

A file type (i.e. PDF) is registered with the system through a process called “LaunchServices.” It’s what associates a file with what opens it. It can even have multiple entries to do things like “Open with Preview” and “Edit with TextEdit” for example. It can’t, however, dynamically change based on a folder because folders aren’t registered with LaunchServices.

Looking at this pragmatically, how would the system know which folders to monitor for application associations of its contents? Folders are created and deleted all the time - it wouldn’t be efficient to manage those relationships.