So piracy is not really an "on" "off" switch. Anyone can pirate anything, with enough determination. The limiter is the determination, as every consumer has some tipping point where they decide "Eh, it's better to pay money than jump through these hoops to pirate," vs. "Eh, it's better to look for pirated material instead of paying this price." It varies from person to person (some folks will never pay, and some will never pirate), most people are somewhere inbetween.
So by making official PDFs available for purchase, this vastly makes piracy much easier, as one person just needs to buy a copy and slap it on a website with some ads to make a quick buck. Right now, someone needs to buy a book, scan each page, then slap it on a website. That limits the number of people who put up a pirated copy, making it harder for a person to find it, making it less likely overall that piracy happens.