Yes, that's possible! This is a very useful feature of BigBlueButton.

Sharing a screen costs a lot of bandwidth ("network traffic") and, for many participants, quickly results in faltering connections. When uploading a presentation, it is made suitable ("converted") for sharing via the web.

In the new version of BigBlueButton, which will be released this spring (2021), multiple presentations can be uploaded, making switching between different presentations easier and faster.