Thanks for your reply,
my problem is that I need to perform two successive crops.
If you check one of the books in the link above, you'll see that most pages are made of a small poem in the middle of a mostly empty page, with a frame near the edge of the page.
I can't use the "remove white margins" options because it will stop at the frames so I need to perform a first crop to remove the frame and the footer, and only then will I be able to crop the white margins.
The problem is that, although after removing the frame, it does not appear on the page anymore, I still can't remove the white space because the frame seems to be still there, outside the margins.
Any help will be greatly appreciated!