If you use bleed in your document, the PDF document will also have a bleed box. Bleed is the amount of color (or any other artwork) that extends beyond the edge of a page. You can use bleed to make sure that, when the document is printed and cut to size (“trimmed”), the ink will be printed to the edge of the page. Even when the page is “mistrimmed”, i.e. cut slightly off the trim marks and a bit more to the “outside” of the page, no white edges will appear on the page.