Which layer contains the board outline in PADS Layout?
I do PCB manufacture. From manufacture point, I suggest you to create a mechanical layer named outline layer and then draw the outline or you can draw the outline on the silkscreen layer. It's easy to recognize the board outline for manufacture engineer.

For visible/invisible, both are fine. But be sure that the manufacture engineer can recognize the board outline. Take rectangle outline for reference, you can draw the visible rectangle outline or you can just draw the four points or the 2 counter points which can present the rectangle outline.