How to share files via screen sharing

Hello you lovely Jitsi people!

A friend and I are trying to use Jitsi for an online English class. However, we can’t figure out how to share all types of files via screen sharing. Sometimes a file appears for sharing in the drop down menu, sometimes it doesn’t. Even if the file is open on our computer, we will sometimes only have the option to share the full screen or the window of the jitsi meeting itself. Can someone explain the prerequisites for sharing a file (word or pdf) so that other users can see it?

Thank you so much for your help!

Victoria