Statistics problems change with every edition. The PDF you find might be the 2012 edition, while your university exam follows the 2022 syllabus. Old print errors might include wrong statistical tables (t-table or f-table), leading to failed calculations.
Just because you don't want to spend ₹500 doesn't mean you have to steal the book. Here are legitimate, often cheaper alternatives to the "free PDF."
Most free PDFs are hand-scanned by previous students. The quality is often terrible: