You might try something like:
Sub InsertDocsAtBookmarks()
Dim ws As Worksheet, objWord As Object, strBkMk
Dim r As Long, c As Long, lRow As Long, strFl As String
Set ws = ActiveWorkbook.Sheets(1)
lRow = ws.UsedRange.Cells.SpecialCells(xlCellTypeLastCell).Row
Set objWord = CreateObject("Word.Application")
With objWord
.Visible = True
.Documents.Open "S:\xxxxxx.docm", , False, False
With .ActiveDocument
For c = 2 To 16
For r = lRow To 8 Step -1
strFl = ws.Cells(r, c).Value: strBkMk = "bookmark" & c - 1
If strFl <> "" Then
If Dir(strFl) <> "" Then
If .Bookmarks.Exists(strBkMk) Then
.Bookmarks(strBkMk).Range.InsertFile strFl
End If
End If
End If
Next
Next
End With
End With
End Sub