Keyboard navigation between widgets of pcmanfm (i.e. when user doesn't use a mouse) should be documented via the Help menu.
Should be documented keyboard shortcuts:
Tab/Shift+Tab - cycle thru widgets (define order!)
F6 - switch between Side pane <-> Folder view
F8 - focus divider between Side pane and Folder view
F10 - activate main menu
Ctrl+L/Alt+D - jump to path bar