You can do that using the "Go FOLDERCONTENT" command. There's an example here:
It has arguments that let you control whether it shows sub-folders and so on.
Go to Settings -> File Types, edit the PDF type and change what the Double-Click event does. The Events tab is Opus-specific while other tabs affect the whole system (unless you define things which only work inside of Opus), so you have the choice of doing both system-wide and Opus-specific file association changes/overrides.