I would like to add additional text in a draft invoice (before posted) but I don't want it in my printout copy. It will be just a internal reference easily to see without go to comments sections.

I would like to add additional text in a draft invoice (before posted) but I don't want it in my printout copy. It will be just a internal reference easily to see without go to comments sections.
You can achieve that but you will have to do some customization around it. There is no out of the box functionality for this.
You can look at customizing the extended text feature you find on items.