ArcMap, storing a variable inside MxDocument?

Is it possible to store a variable inside the MxDocument itself like you would for AutoCAD? I ask because we want to create our own scale factor for a drawing/document that is set by the user and just our custom tools will use it. I haven't really been able to find anything about there from searching online and it seems like something that would be very useful and used often.

If it is possible, does anyone have an example or a link to an example of this for me to go off?

Thanks for the help.