I guess I should have expanded: a watermark is useful to reduce the number of possibilities of who could have released it. Over time, the more information is gained, the closer you are to a singleton (the result of the intersection of sets). Whether that takes 1 or 10 watermarks does not make a real difference over time, especially when specialized websites release thousands of such PDFs.
So if you have a watermark that associate files with the time they have been downloaded, and download it from multiple networks etc. at the same time, and remove the diff from these versions, you'll still have the time-based watermark in the resulting file, so you'll leak when you downloaded it.
Problem is you only have to fail once to leak information, and there are only a finite number of bits of information you can leak before your identity is known.
So if you have a watermark that associate files with the time they have been downloaded, and download it from multiple networks etc. at the same time, and remove the diff from these versions, you'll still have the time-based watermark in the resulting file, so you'll leak when you downloaded it.