Loop through subfolders to join pdf in each folder

Dear kind Developers, hereby i have a folder which contain subfolders like below:

each folder contains subfolder like below
and each subfolder contains pdf which i need to join:
the same goes to every other folders. i need to loop through each subfolders and join pdf on each folder and store that pdf on that folder itself. i can use the join pdf activity but i am confused how to loop through each subfolders to join the pdfs. thank you so much in advance.

Refer this workflow.
Folder.xaml (5.8 KB)