how to use an already open document from another module? - vba

Question

i write in a word.document. When it is necessary i write a paragraph in another document and when i finish this document is closed. then i try to copy this paragraph to the first document which remains open.
I try some things but in vain.
when i do this:
Dim wdDocMain As Word.Document
Set wdDocMain = ActiveDocument
i receive the message run time error 4248 This command is not available because no document is open.
when i do this
Dim wdDocMain As Word.Document
Set wdDocMain = wdApp.Documents.Open(FileName:=pathmaindoc,_ ReadOnly:=False, Visible:=True)
wdDocMain.Activate
i receive the message run time error 91 object variable or with block variable not set.
i find a solution.
To close the open document and then to reopen it like this:
Set wdDocMain = wdApp.Documents.Open(FileName:=pathmaindoc,_ ReadOnly:=False, Visible:=True)
but of cource it isnt the best.
Edit.
I would like to thank you in advance for your comments.
i share more lines from my code as Timothy Rylatt suggested.
In my main sub:
Dim wdapp As Word.Application
Dim wdDoc As Word.Document
Set wdapp = New Word.Application
Set wdDoc = wdapp.Documents.Open(FileName:=PathName_HiveDown_Creation,_ ReadOnly:=False, Visible:=True)
wdapp.Visible = True
when a condition is met then i call a module (copyparafromtemplatetomain):
pathmaindoc = PathName_HiveDown_Creation
pathtemplatedoc = PathName_Template
'wdDoc.Close savechanges:=True
copyparafromtemplatetomain
Public wdapp As Word.Application
Public wdDocMain As Word.Document
Sub copyparafromtemplatetomain(pathmaindoc As Variant, pathtemplatedoc As Variant, paramain As Variant, paratemplate As Variant, index As Integer)
Dim wdDocTemplate As Word.Document
Dim PathName_MainDoc As Variant
PathName_MainDoc = pathmaindoc
Set wdDocMain = wdapp.ActiveDocument
here, i receive run time error 91.
the only way to run is when i close the doc. in the first sub