If this file is to be downloaded by attendees, you can upload it through the session details. Just click Advanced Settings > + Manage Document.

As an admin, you can go to Event Content > Speakers and click Copy Speaker Hub Link. When you access the link, you'll see a dropdown that let's you pick which speaker you want to view. From there click + Add Documents on the session to upload the files. Since speakers have access to the speaker hub, they can also upload their own files.

If this file is to be presented on screen, it cannot be uploaded. The speaker or the moderator will have to share their screen so that attendees can see the presentation. From the Studio, they will have to click the Share icon. Then select the share screen option, and select the file or the screen they want to show.