[viraptor]

For PDF you can do this in a much more subtle way. In a typical block of text every individual letter comes with its own kerning adjustment. You can adjust those in a way that's invisible to the reader but still allows fingerprinting. There's probably 1000 different options too - don't think of moving words as in swapping positions in a sentence. (I know parent suggested it, but that's silly)

[matheusmoreira]

These probably wouldn't survive extraction of the pure text, would they?

[out_of_protocol]

Replacing characters with identical-looking unicode chars, adding extra spaces here and there, adding newlines (and more spaces :)), adding random typos, use dictionary with "safe" word/phrase replacements etc. And don't forget about formulas, charts etc - pure text version is not too useful on its own

[hdjjhhvvhga]

If you deal with fiction and the like where you basically have just text then I think that's correct: it would be trivial to detect the watermarks in various copies by simply comparing them. I was dealing with PDFs containing tables, formulas, illustrations, etc., so a plain-text version would be unusable.

[snovv_crash]

Randomly choose 3 big paragraphs in the entire ebook to add an extra newline in the middle of at the end of a random sentence. This would be my choice if I had to do some kind of invisible watermarking, at least.

[hdjjhhvvhga]

This is one of the many things that could be trivially detected and fixed when you have multiple watermarked copies of the same file.