Downloadable manual

If you’re looking for something like a pdf or an epub, I don’t think there is one.

If you have Pianoteq installed, there is a single HTML file at:

"C:\Program Files\Modartt\Pianoteq 9\Documentation\pianoteq-english.html"

(the exact location and name will vary if you’ve installed in a non-standard location or have a non-English version). There should be a shortcut to it on your Start menu. That file will open offline, so there’s no need for an Internet connection to read it. As far as I can tell, it has the same content as the online version, though the table of contents is inline near the beginning rather than being loaded on demand in a sidebar.

If you don’t have Pianoteq installed and you want to read the documentation offline, just go to:
https://www.modartt.com/user_manual
and choose the language you want, then use whatever is your browser’s command to Save As / Webpage-Complete. It is just one long page, so there’s no need to worry about downloading any other pages. For some reason I have not taken the time to figure out, the table of contents does not show if you make a local copy of the online page.