But probably the main security here is just using the chrome pdf viewer instead of the adobe one. Which you can do without google drive. The browser PDF viewers ignore all the strange and risky parts of the PDF spec that would likely be exploited.

This is a very common assumption that turns out to be false.

There are Tardos probabilistic codes (see the paper I linked) which have the watermark scale as the square of the traitor count.

For example, with a watermark of just 400 bits, 4 traitors (who try their best to corrupt the watermark) will stand out enough to merit investigation and with 800 bits be accused without any doubt. This is for a binary alphabet, with text you can generate a bigger alphabet and have shorter watermarks.

These are typically intended for tracing pirated content, so they carry the so-called Marking Assumption (if given two or more versions of a piece of content, you must choose one. A pirate isn't going to corrupt or remove a piece of video, that would be unsuitable for leaking). So it would likely be possible to get better results with documents, may require larger watermarks to get such traitors reliably.

The Chrome PDF parser, originating from Foxit (now open-sourced as PDFium), has been the source of many exploits in Chrome itself over the years.