We are in the initial steps of installing Portal.
We have a server we are installing Portal on and at no point in any documentation does it state that IIS is even required? I've been working with ArcGIS for Server for years now so this is a huge leap forward and the documentation is anything but straight forward.
From what it looks like, we need IIS on the server with the Web Adaptor. Other than that, Portal can exist on a server without IIS.
Does anyone have anything that might help me on this?
Portal requires some sort of "front-end", for example a reverse proxy or the Web Adaptor, (which in itself is a reverse proxy). On Windows, the Web Adaptor requires IIS as it runs within the IIS instance, so if you plan on using the Web Adaptor, you'll need IIS on whatever machine you plan on installing the Web Adaptor on.