Some PDF editions I find online are fitted for Euro-sized paper and are too tight on 8.5x11. I wanted to to introduce some margin here but the regular page crop tool doesn't work. So here is the answer.

Reprint to PDF while setting the page size to 7.5x10. Click the option to shrink to fit. Once that is done, you reprint the new file to 8.5x11, this time without option to fit the page to PDF. Voila! a margin appears.

Anyway, I post this because I found it sort of tricky. Glad to know there is a solution.