If you actually merge to a new document, you will have fixed content. What
you are doing is what I do, that is, merging to the screen. What you get
when you reopen depends on your database.

I've added buttons to my merge toolbar that triggers the following macros.

Sub FieldsLockAllStory()
'   All Story Field Locker
'   Written by Charles Kyle Kenyon 9 December 2004
'   repaired with help from Jezebel 10 December 2004
   Dim oStory As Range
   On Error Resume Next
   For Each oStory In ActiveDocument.StoryRanges
       Do
           oStory.Fields.Locked = True
           Set oStory = oStory.Next
       Loop Until oStory Is Nothing
   Next
End Sub
Sub FieldsUnLockAllStory()
'   All Story Field Unlocker
'   Written by Charles Kyle Kenyon 9 December 2004
'   repaired with help from Jezebel 10 December 2004
   Dim oStory As Range
   On Error Resume Next
   For Each oStory In ActiveDocument.StoryRanges
       Do
           oStory.Fields.Locked = False
           Set oStory = oStory.Next
       Loop Until oStory Is Nothing
   Next
End Sub

I use a modified version of the padlock button from the forms toolbar for
these (changed color on both and open lock on the one to unlock).

Note if you have a Page X of Y in your header/footer, you'll want to
manually unlock that field or it will be wrong.

Otherwise, you can print as a pdf file.