Merged documents are not connected to the data source.The fields are substituted for the data they represent.
If you want to send individual letters - see E-Mail Merge Add-in
If you want to send the merge document only then change the merge document type to normal Word Document. It will just have the fields unlinked from the data. If the coworker needs to merge that document, send him the data source and let him change the document merge type to Letters and run the merge.

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Graham Mayor - MS MVP (Word) (2002-2019)
Visit my web site for more programming tips and ready made processes www.gmayor.com