On the Document list screen, select the document type for sales documents (i.e. Invoices, Credit notes or Quotes) or purchase documents (i.e. Purchases, Supplier returns or Orders).

If you need to select multiple documents, press the Ctrl key and click on each document you wish to copy.

Right-click and select the Documents → Copy document option on the contest menu. Exact copies of the selected documents will be created.

Once this is done, you may edit / post these documents.