Hi PaulW,

very likely, a datafile is not a Word-doc at all,
but plain text.

If so, then you just need a programming language
and basic programming skills.
VBA comes in handy. Yes.
Whether it is Word-VBA or Excel-VBA or whatever,
doesn't matter much.

Have a very close look at this sample code:

Sub SplitTxt()
Dim lngLin As Long     ' line number
Dim strTmp As String   ' string from a line
Dim lngDcm As Long     ' number of created doc
Dim strTrg As String   ' target path
strTrg = "c:\test\text\"
lngDcm = 2
' ---------------------------------------------
Open "c:\test\thisDoc-001.txt" For Input As #1
Open strTrg & Format(lngDcm, "0000") & ".txt" For Output As #2
While Not EOF(1)
  lngLin = lngLin + 1
  Input #1, strTmp
  Print #2, strTmp
  If lngLin Mod 1000 = 0 Then
     lngDcm = lngDcm + 1
     lngLin = 0
     Close #2
     Open strTrg & Format(lngDcm, "0000") & ".txt" For Output As #2
  End If
Wend

Close #1
Close #2
End Sub

Which could be improved in many ways.

It opens a text-file (source).
Then another file (target), with initial name "0001".
Reads line by line from the source,
whereby it counts the lines,
and writes the lines to the target.
If 1000 lines were written,
then it closes the target file,
and starts counting from 0 again.

It increments the number of the target file by 1,
and does so till the end of the source file was reached.

Not that simple, but basic nevertheless and essential.