I have a 7zip password protected folder with several also password protected pdf files with selective sync unchecked, (double protection and not in the Dropbox folder in my PC). I cannot open them with the dsektop app, buy they can be opened from my android smartphone without having to download, just download and install the RAR app from Google play. it works fine!! It's a pity that Dropbox doesn't provide this option for the Dropbox desktop App.
Thank you all. I have been spending hours trying to find out why I can not open the password protected files on my laptop while my phone can open them!! And my answer was here !! I searched the dropbox site for hours but no information there !!!