If you want to get the horizontal scrollbar visible, you will have to use IN WINDOW, not WINDOW. Here is a slight adaptation of your code that should work:

LOCAL oPreviewForm, cFile
oPreviewForm = NEWOBJECT('PreviewForm')
cFile = GETFILE("frx")

REPORT FORM (cFile) IN WINDOW PREVIEWFORM PREVIEW 

oPreviewForm.Release


DEFINE CLASS previewform AS form

	Height = 250
	Width = 375
	Desktop = .F.
	ShowWindow = 2
	DoCreate = .T.
	AutoCenter = .T.
	BorderStyle = 3
	Caption = "Print Preview Window"
	ControlBox = .F.
	Closable = .F.
	MinButton = .F.
	Movable = .F.
	WindowType = 0
	WindowState = 2
	AlwaysOnTop = .F.
	Name = "previewform"


	PROCEDURE Deactivate
		DODEFAULT()
		IF WEXIST('Report Designer')
			ZOOM WINDOW "Report Designer" MAX
			MOUSE DBLCLICK AT -0.1,0.1 WINDOW "Report Designer"
		ENDIF
		IF WEXIST('Print Preview')
			MOUSE DBLCLICK AT -0.1,0.1 WINDOW "Print Preview"
		ENDIF

	PROCEDURE Init
        DODEFAULT()
        THIS.SHOW()
        KEYBOARD 'L'   && WORKS!  75% is perfect
                
	ENDPROC


	PROCEDURE Activate
		DODEFAULT()
		IF WEXIST('Report Designer')
			ZOOM WINDOW 'Report Designer' MAX
		ENDIF
		
		IF WEXIST('Print Preview')
			ZOOM WINDOW 'Print Preview' MAX
		ENDIF
		
	ENDPROC


ENDDEFINE