The Kindle version is often on sale. While Kindle uses a proprietary format (.azw3), you can read it on any device via the free Kindle app (iOS, Android, PC, Mac).
If you insist on trying to find a free PDF against our advice, at least protect yourself:
Honestly, the safest “free” method is the library app. It’s legal, instant, and respects the author who gave you this brilliant, anxiety-inducing story. The Devil Wears Prada Book Pdfl
A common misconception: Older books are free. The Devil Wears Prada was published in 2003. It is not old. Do not look for it on Project Gutenberg or Internet Archive’s lending library (except for controlled digital lending, which requires a waitlist).
Once you finish the original, you’ll want more. Luckily, Weisberger wrote follow-ups: The Kindle version is often on sale
All of these are available via the same legal eBook channels.
If you only know the story from the 2006 film starring Anne Hathaway and Meryl Streep, you are missing half the story. The book is actually quite different (and darker in some ways). Honestly, the safest “free” method is the library app
You need the raw, unfiltered text to get the full revenge fantasy.