Thanks for the feedback!
The document page backend already supports this, the missing part is the user interface. We started working on this and then realized that this feature ties in with document splitting and document merging. So it was back to the drawing board to come up with a user interface language (widgets) to solve all cases.
This is how it is currently implemented on a development branch.
You open two documents using a split panel view and then drag and drop the pages you wish to either move or copy from one document to the other. The documents don’t need to be the same file type, you can move pages from a word processor into a PDF.
This feature requires frontend JavaScript code that has not been merged or fully tested. Until that blocking issue is resolved this cannot be merged and shipped.