Adding Google Docs to Multiple Folders

Q: Can I add a single Google Doc to more than one folder?
A: Yes!  Rather than dragging the file into a folder, check the box next to the file, then click the folder icon at the top.  Hold down the Ctrl key (Windows) or Command key (Mac) as you check each folder to add the doc to.

Note that you are not making copies of the file when doing this.  It is the same document going into each folder, so any changes you make in one place will carry over to the others.