Dear Candace,
Welcome to the forum here.
Based on my test, the HTML file should be opened directly.
May I know whether all users in your environment can't see the page content? Are there some embeded iframe or links on the page content?
If you create a simple HTML page, save it on the library and open the file, what's the result?
Regarding "I would like users to access directly on the 2013 Modern SharePoint site.", are you using SharePoint Server or SharePoint Online? If you are using SharePoint Server, we may have limited resources on coping with SharePoint Server related issues and questions, I suggest you post a new thread on our specific support channel office-sharepoint-server-itpro - Microsoft Q&A for expert help.
If you are using SharePoint Online, welcome to share the HTML file you are using for us to do atest. If the file and the file link can be shared here directly, just provide the file here. If no, please let us know and we will take the next step to collect the file.
Thanks for your effort and time!
Sincerely,
Cliff | Microsoft Community Moderator