Change default app for all files in a 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.