jdsaurborn,
Got it. Since, we're able to isolate this behavior to the Calendar app of the Mac, we suggest to restart your Mac in safe mode. This helps to perform a basic check of your startup disk, deletes some system caches and more. Here's a link with additional details and the proper steps to restart the Mac in safe mode: How to use safe mode on your Mac
Once the Mac is in safe mode, please restart your Mac as normal and test to see if your invitees can view the attachments from a calendar invite.
In addition, please ensure that you're on the latest version of the macOS operating system. This helps provide software improvements that can help: How to update the software on your Mac
Kind regards.