First things first, you might want to check if 'The Pact' by Jodi Picoult is actually the book you're after, or if you mean the 'Pact' series by someone else – I've seen that mix-up happen before. The best route for a guaranteed safe download is always the author's or publisher's website; Penguin Random House handles Picoult's work, and they often have direct links to major retailers.
For digital, your library's OverDrive or Libby app is a solid, completely legal option. You just need a card, wait times vary, but it's free. Amazon Kindle, Google Play Books, and Apple Books are the obvious paid spots – they're not free, but the files are clean, and you own the license. Honestly, skipping random PDF searches saves you from malware and supports the writers.
Sometimes the official sites have excerpts or first chapters for free download, which is a good way to sample before you buy. I grabbed a sample of 'The Book of Two Ways' that way last year before committing.