thanks for your reply

however, there are various software with ability to view pdf etc files in a pane inside them

maybe EXCEL can do this? or any VBA code or something?

I basically need an outliner with ability to store files in each of the treeview items and quickly view them (it's ok if editing them is not possible)

and the universality of MS Office products makes me want to implement this outliner inside these Word, Excel etc, because all PCs can read them, without any additional software