If you can't share another application (pdf, image) using screen sharing, it may be because you're in full-screen mode on Safari. If you are not in full-screen mode you can check this article to see how you can share your screen and application.

Do not use Brainstork full-screen mode!

If you want to use the screen-sharing function, you should use Brainstork with minimized version, not a full-screen version.

Safari

When you try to start screen sharing on the full-screen mode you will see a warning from Safari.

Click the screen to exit fullscreen and start sharing.