Just started with ZD

There are pdf files which i'd like to transfer to the briefcase. However when I click on the tab, the dialogue box does not open. So I tried working with my other files and it worked and uploaded to files as 2MB.

However there are also pdf files as small as 152kb wherein I'd like to transfer to the briefcase but the briefcase tab is just dead.

What am I missing?

Thank you in advance for the help.