Robohelp 3D CAD files

As Amber mentioned if you get the CAD output which can be rendered in HTML pages then it will work in RoboHelp. E.g. https://www.cadasio.com/ has outputs which work in RoboHelp.

I think the answer is "it depends". 

Do you want people to view 3D CAD drawings in your help? Then I don't think you can. I believe you'd have to export to a supported web format. It's possible something like WebGL might allow it, but I couldn't say for certain and I don't know what would be involved to implement, but maybe it'll help your search. If there is a javascript library that allows it, it will be easier to implement in New UI than in Classic.

If you want people to be able to download CAD files for use in AutoCAD on their computer, then the answer is 'yes I think you can'. For Classic I think you'd just need to add them as baggage files and link to them. For New UI, just copy and paste the files into folder and link to them. I'm not sure what browser security settings might do with a CAD file, but in theory they should just save like any other file you might link to.