Documents available to share

When I click the green button to share screen, the things that are available to share seem random, and I can't find how to select the document I want.

My documents are on Dropbox, but when I sign in to Dropbox, and select a file to share, it opens in a strange and unhelpful format, that one can't scroll down.

So I tried creating a document specifically to share, kept it out of Dropbox and placed it on my desktop.

But it doesn't appear as an option to share, and I can't find how I select it.