Literally the next sentence:

The matching document type (in this case, PDF) will subsequently automatically trigger Google Gemini for all future files of the same type opened within Google Drive.

So documents you didn’t tell it to look at.

Yes, they said it’s coming soon via software update.

https://9to5mac.com/2024/03/04/14-inch-m3-macbook-pro-multi-display-support/