Referencing a Word document that is already open

[macropod]

Try:

Dim AppWd As New Word.Application
 Dim WdDoc As Word.Document, WdRng As Word.Range
  
 Set WdDoc = AppWd.Documents.Open("C:/test.docm")

You can now reference the document any time you need, via WdDoc. For example:
Set WdRng = WdDoc.Sections(1).Range
sets WdRng to span only the first Section in what may or may not be a multi-Section document. Having dealt with that, you might want to do something to the entire document, for which you'd reset WdRng:
Set WdRng = WdDoc.Range