This is very useful, and I would add the following if appropriate:
Copy/Paste doesn't appear work when two files are open in two separate QCad windows, but it works within same the QCad window (open one document, and File->Open second document).
Within the "same" document Copy/Paste thing works as folows:
1. Edit->Copy (or Ctrl+C)
2. select objects you want to copy
3. on toolbar press right arrow
4. specify reference point by clicking or entering coordinates
(objects are copied)
1. Edit->Paste (or Ctrl+V)
2. specify point where objects will be pasted (by clicking or entering coordinates).