Preview.app with .pdf: crop box view in slideshow

Since the media box (as you may know) is the size of the largest page in the PDF file, regardless of placement of objects in the PDF. The crop box is always a subsection of this (or the same size, but never larger), defined by you. It should be the part of the PDF page (media box) that contains the text, pictures, etc. that you want the person reading your PDF to see.

For example, when composing the layout for print publication, you would want to set the media box to be slightly larger than the media you will be printing on. You would then set the crop box to be the precise size of the final, printed media. If you were printing on normal 8.5" x 11" paper then media box would be larger than 8.5" x 11", and the crop box to 8.5 " x 11". This way, you can position pictures and text and whatnot right up to the edge of the printed page. This will also allow you to see whether anything is positioned wrong and is hanging over the edge of the crop box (which would mean it'll be cut off when printed".

So basically, what I meant to say is: the crop box isn't meant for changing how a PDF is displayed; it's meant to control what prints out, and what doesn't, when printing to paper. Microsoft Excel does the same thing -- when looking at a spreadsheet on the computer, you see all cells. If you want to only print a portion of the spreadsheet then you'd use the "set print area" option. Setting the print area isn't the same as just printing a single page or selection of pages, although in some situations they may produce similar printed pages.