PDFPageMBS.drawRect(box as Integer, g as NSGraphicsMBS)

Sub drawRect(box as Integer, g as NSGraphicsMBS)
  
  if g.Valid then
    System.DebugLog "is valid"
  else
    System.DebugLog "not valid"
  end if
  
  g.SetColorRGB 0,1,0
  
  g.drawRect 200,200,200,200
  
  g.SetColorRGB 1,0,0
  
  g.fillRect 100,100,100,100
End Sub

Do not store the graphics reference as it is only valid in this event.
Use the kPDFDisplayBox* constants for the box value.
You can draw another PDF page here with Draw command.

To avoid drawRect causing StackOverflowException when showing in a control, please have PDF produced as file or MemoryBlock, then read it back to a new PDFDocumentMBS object and show that in a control.