[scribus] Bleed.

[Peter Nermander]

> You state: "The PDF contains bleed and trim boxes that the
> printers imposition software adheres to, and crop marks are added for
> the final spreads."

> I don't see these marks. Are they invisible to a viewer, then?

Yes, they are no visible marks but coordinates for the corners of the boxes.

Adobe Acrobat (not Reader) can display (and manipulate) them. I'm not
sure about Adobe Reader.

You can read more about them for example here:
http://www.prepressure.com/pdf/basics/page_boxes
(I did a quick google)

/Peter