Most free PDFs floating around the internet are scanned copies of old editions (e.g., 2004, 2007, or 2010). Pressure vessel technology evolves. Critical safety rules change. If you design a vessel using a 2010 edition instead of the 2025 edition, your vessel could fail catastrophically. Moreover, the scanned images often have missing tables, distorted formulas, and faded text that make the code unusable.