It works for me:
- Find a page that will allow you to find the margins
- Select the crop box with dashed-box tool (you can drag and resize the box)
- Select Thumnails for the sidebar and click one of the pages
- Hit Command-A to move the crop box selection to all pages
- Hit Command-K to crop
- Save
Not nearly as nice as soPdf (Windows only, as far as I know), and the Mac will sometimes balloon the size of the PDF, but it works well enough.