Finding a legal, free PDF of 'Atonement' can feel like a treasure hunt that mostly leads to dead ends. Ian McEwan's novel is still under copyright, so publishers and retailers are the primary gatekeepers. Your best shot for a legitimate free copy is likely through a public library system's digital lending service, like Libby or OverDrive. You need a library card, of course, but once you have that, you can borrow the ebook, which isn't a PDF but reads just fine on most devices. It's not ownership, but it's free and legal.
I got my copy that way a few years back when my book club picked it. The waitlist was a couple of weeks, but it was worth it. Always be wary of sites promising 'free PDF downloads'—they're almost always pirated material, which isn't fair to the author or publisher. Sometimes universities with open access projects have older works, but for contemporary fiction like this, the library is really the main ethical avenue.