So, what happens when someone's watermarked copy starts showing up online? Will you be assuming the only way it got out was abuse by the intended customer and send in the lawyers? Did you notify your customers before they paid anything of this risk they're taking?
Most likely I'd send a polite e-mail letting them know that I saw this and reminding them that they aren't supposed to share the file. The expense of a lawyer wouldn't be worthwhile. You seem to assume nefarious intent. My goal is simply to be able to let people buy a DRM-free copy of the PDF with a mild disincentive against sharing.
Not necessarily "nefarious", but I certainly don't trust everyone to behave reasonably or logically. The very act of putting the watermark in will plant that idea in at least some of your customer's heads. And since you didn't answer my question about whether you informed customers of it before they made a purchase, I think it's safe to assume you didn't. Even if all of your customers trust you to act reasonably, you could die tomorrow and all of those rights will get transferred to someone else who may not be as trustworthy.
