If a watermarked PDF ended up on the internet it does not necessarily mean that the person who purchased the said PDF leaked it themself or did anything wrong f

If a watermarked PDF ended up on the internet it does not necessarily mean that the person who purchased the said PDF leaked it themself or did anything wrong for that matter. Computers are hacked, stolen all the time. At Universities machines in the lab are oftentimes shared and document dumps exist on shared partitions. In a court of law (at least in the USA) the burden of proof is on the plaintiff. It could be expensive and difficult to prove that the PDF purchaser did upload it themselves and broke the law. Similar to the music and movie pirates - going mostly after big fish.