reading the readme_changelog.txt I see:

"- added English & Brazilian Portuguese manuals to the game folder"

But I can't find the manual anywhere

I can confirm this statement.

Well, this log most likely is not created by GOG, but rather the developer. GOG usually provides the manuals as a bonus and there are none here for Trine 2 (which is a shame), so we can assume GOG does not have the manuals at all and therefore did not include them in the game's package.