Currently, I have two solutions in my organization.
Provider Hosted App developed in .Net MVC with C# languange and hosted in Azure as an App service. Also, we have deployed this app in the SharePoint app catalog. So, ultimately our users are accessing the Dot Net application through the SharePoint Online platform.
An Enterprise Content Management System on SharePoint Online where we have multiple document libraries in different subsites to store pdf files.
I hope the above information is enough to give you an idea of my current environment and the solutions that we are using. My Requirement: I am trying to open/embed SharePoint online document library webpart in a popup window in Dot Net application but when I do this, it gives me this error "Refused to display 'https //login.microsoftonline.com in a frame because it set 'X-Frame-Options' to 'deny'."