There’s no one straightforward answer to that, because it really depends on where you’re looking and what the author/publisher’s distribution strategy is. That title sounds like a contemporary romance or a steamy billionaire romance serial, right? Those are often part of Kindle Unlimited or other subscription services as a core part of their business model. The author might offer the first few chapters for free on their website or through a newsletter sign-up to hook readers, but the full PDF? If it's a paid title from a platform like Amazon, Apple Books, or Kobo, a complete free PDF floating around on some ‘free ebook download’ site is almost certainly not legal. Those sites are notorious for pirating content, even if they dress it up as a ‘free library’.
A lot of readers get tripped up by the difference between ‘free with subscription’ and ‘free to own’. If the book is in Kindle Unlimited, you can read it for ‘free’ with your monthly subscription fee, but you don’t get a downloadable PDF to keep. Same with apps like Radish or Dreame where you unlock chapters with time or tokens. That’s a licensed, legal way to read without a direct per-book charge, but it’s not the same as finding a standalone PDF file. The only time a full PDF might be legally free is if the author themselves has chosen to release it as a permanent freebie on platforms like Prolific Works or as a direct download from their site, often as a lead magnet. I’d check the author’s official social media or website first. If it’s not promoted there, any other source is sketchy at best.