How do I attach a document to an IFC file in S-INFO?

Frankly speaking, no way. The IFC format does not support attachment of other files.
But S-INFO allows you to automatically create links to attached files when exporting IFC. All you need is: 

1. Attach a document to an element in properties. 
2. Export selected elements to IFC. 
3. Open IFC in any viewer.
4. Open the properties of the element and follow the link. 
5. Enjoy access to files in the browser, an example of which is in the figure.