Here's the story. A middle-size publisher contacted me about the problem they had: someone was distributing their PDFs online using various services, but mainly Scribd. At that time (it was maybe 2012?) it worked like this: the attacker released a document, notified his group, and they started downloading. The publisher immediately filed the DMCA copyright infringement notification and it was handled within 3-4 days - more than enough for people to download the "release". It was going on for a long time and Scribd refused to remove the account of the attacker.[0] After a couple of months they finally gave in and deleted it but of course a new account was immediately created. It was not a mouse-and-cat game, but more like mouse-and-turtle.

So when they asked my help I told them, you don't have a copyright problem, you have a SEO problem - since people looking for their publications could find them easily online with the stolen copies sometimes appearing higher on SERP than their own. They said they would address that and they somehow came to terms with the fact that the past documents are lost, but they would like to avoid this situation in the future.

I checked some of their publications. They were moderately priced and the content was quite interesting. Many of their customers were very supportive - actually they were sending reports when they noticed the pirated copies. And they weren't saying "Why should I pay for your books when I can get them online?" but rather "Please take care of protecting your content since we want you to survive and publish more books". So, to answer your question, I was very happy to work for them, and I would do it again.

[0] I was told they changed their approach later and started to collaborate with publishers.

As a customer I want a thriving, commercially viable, decentralised publishing industry. I don't want DRM encumbered formats, or a single vendor, or a point of failure that prevents me from reading books I've purchased.

If my email address embedded in the PDF enables that then I think that's reasonably balanced ethics arithmetic.

(edit - I consider public-funded scholarly research to be a different matter to private purchases of commercial books such as fiction or trade textbooks)

It wouldn't work since we were allowed to manipulate certain aspects of text, too, to a certain extent, in a way that looked pretty much statistically random. That is, by comparing several copies of the same file, the bits were very much different, and the differences were distributed over the whole document. One of the main aims was to make sure the watermark gets preserved over various transformations, and we automatically tested each new document to make sure it works.

"I would just" [do this slightly more complicated transformation] really misses the point. Nobody disputes the fact that you could turn any given human-readable pdf into an untraceable version. E.g. you could copy it all out longhand and make sketches of all the plots. Certainly it would work. The point is that as the necessary transformation becomes more complicated/more work, the fraction of the population that will actually do it will vanish, and the surveillance scheme will be effective again.

I'm curious what a legal case would look like around steganography. "Trust that our system says that this string is embedded". Or would the prosecution be obliged to divulge the algorithm.