A lot of folks seem to assume any PDF they find floating around is the full deal, but that's a risky bet. I grabbed a PDF labeled 'Before We Were Strangers' last year thinking it was the whole book, only to realize halfway through it skipped a whole section about the protagonist's college years. It just jumped timelines, and I had to hunt down a physical copy from the library to fill in the gaps.
Unless you're downloading directly from a known publisher's site, an official retailer, or a library service like Libby, there's no guarantee. Those unofficial PDFs are often compiled from scrapped web text or pirated scans, and abridgments or missing chapters happen all the time. I'd say if reading the complete story matters, the PDF alone isn't trustworthy enough without verifying its source against the ISBN or page count of the official edition.