In addition to the Documents Copy document option on the context menu of the "Document list" screen, to create exact copies of the selected document(s) of a specific document type, you may also copy any of the documents to another document type.  

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

Right-click and select Documents Copy to Invoices, Credit notes, Quotes, Purchases, Supplier returns or Orders option on the context menu. New documents for the selected document type will be created.

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

documents-context-menu-copy-documents-to-type