Without IP your favourite books, movies, TV shows, music and video games would not exist.

You are arguing that Elsevier shouldn't exist at all, or needs to be forcibly changed into something more fair and more free. I 100% agree with this.

But my point was in general, not about Elsevier but about all digital publications of any kind. This includes indie publications and indie games. If an indie developer makes a game, and it gets bought maybe 20 copies but pirated thousands of times, do you still say "fuck that" to figuring out which "customer" shared the game?

I agree with "fuck that" to huge publishers, and by all means pirate all their shit, but smaller guys need some way to safeguard themselves, and there's no way to decide that small guys can use a certain tool and big guys cannot.

It can be used to spy on any decent scientist who will send papers his/hers/theirs institution has access to, but their friend doesn't.

By "spy" I mean things like: know how many times I've read the PDF, when I've opened it, which parts of it I've read most, what program I used to open the PDF, how many copies of the PDF I've made, how many people I've emailed it to, etc. etc. etc.

This technique can do none of that. The only thing it can do is: if someone uploads the PDF to a mass sharing network, and an employee of the publisher downloads it from that mass sharing network and compares this metadata with the internal database, then they can see which of their users originally downloaded it and when they originally downloaded the PDF. It tells them nothing about how it got there. Maybe the original user shared it with 20 of their colleagues (a legitimate use of a downloaded PDF), and one of those colleagues uploaded that file to the mass sharing site without telling the original downloader. It doesn't prove one way or the other. It's an extremely small amount of information that's only useful for catching systemic uploaders, e.g. a single user who has uploaded hundreds or thousands of PDFs that they downloaded from the publisher using the same account.

And a savvy user can always strip that metadata out.

As a reminder, ...

All true, and fucked up, but it's not related to what I was talking about. I was talking about the general use of this technique.

That's true. I was actually thinking/talking about this practice in general, not specifically with regards to Elsevier.

I definitely agree that scientific journals as they are today are unacceptable.

It would be pretty trivial for a script to automatically detect and delete tags like this, I would think. Diff two versions of the file and swap all diff characters to any non-display character.