Is the user manual only visible because someone in the publications division accidentally published it too early?
If the product has been publicly announced by the company either via some form of statement, press release, etc then it should be public domain.
If a web search crawler happened to find an internal page that wasn’t linked from anywhere, then that may not really be public domain.
The easiest way to answer this question is ask someone if you can put it in your portfolio now, either your boss if it’s an internal project or someone on the companies marketing team if you’re a freelancer/consultant.