We encourage you to create one community for all your similar events, and make this community public. If you create content about the different events and make people use the forum, you would generate engagement and it would be easier to announce new events.
If you want to share some private documents with just the attendees from a specific event, remember you can create a private area on your website by just modifying the visibility of the modules.
Please read engagement tips for more detailed info. How to Build my Community/s?