That's a surprisingly tricky one to pin down. 'In Sheep's Clothing' is a common title, and I spent a good twenty minutes on this last week. If you mean the one about manipulative people by George K. Simon Jr., it's a non-fiction psychology book. You're unlikely to find a free, legal PDF of the full book because it's still commercially in print and quite popular. The publisher wants you to buy it. Your best legal route is through library services or ebook retailers. My local library's OverDrive had the ebook, which lets you download for offline reading through their app. Amazon, Google Play Books, and Kobo sell the ebook, and you can download it to their respective apps after purchase. I got mine on Kobo during a sale. Sometimes, the publisher or author's site might offer a sample chapter PDF to draw you in, but that's usually it. I remember checking the author's website and only finding links to major retailers. It's a bit of a hunt for a straightforward book.
On a related note, I'd be cautious of any site promising a full PDF download for free—those are almost always pirated copies hosted on shady file-sharing sites. Not only is it illegal and doesn't support the author, but you also risk malware. The official channels might cost a few dollars, but you get a clean, safe file and the satisfaction of a proper purchase. The wait for a library copy can be worth it if you're patient.