To do this for every file, you'll need to use an Application-level event.
Put this code in an add-in that you place in your XLStart directory:
First, create a class module, named, say, MyWindowClass
In the class module, enter:
Public WithEvents oApp As Application
Private Sub oApp_SheetActivate(ByVal Sh As Object)
ActiveWindow.Zoom = 90
End Sub
In a regular code module, enter:
Public clsMyWindow As MyWindowClass
substituting your class name for MyWindowClass, if necessary.
Finally, in the ThisWorkbook code module, enter
Private Sub Workbook_Open()
Set clsMyWindow = New MyWindowClass
End Sub