IMHO, you'd do better to use a 'proper' PDF to Doc conversion package. Be that as it may, if you insert the 'PDF' page image, then immediately run the following macro, it will be resized to the page size and postioned behind the text.
Code:
Sub FormatPDFPage()
Application.ScreenUpdating = False
With Selection
If .InlineShapes.Count > 0 Then .InlineShapes(1).ConvertToShape
If .ShapeRange.Count = 0 Then Exit Sub
With .ShapeRange(1)
.LockAspectRatio = msoTrue
.Width = Selection.Sections(1).PageSetup.PageWidth
If .Height > Selection.Sections(1).PageSetup.PageHeight Then
.Height = Selection.Sections(1).PageSetup.PageHeight
End If
.WrapFormat.Type = wdWrapBehind
End With
End With
Application.ScreenUpdating = True
End Sub