The official Plesk Panel help documentation is available directly within the Plesk Panel user interface.
From any page in the Plesk Panel, click the ? button in the top-right of the page to open the Plesk Panel Inline Help:
Context relevant help will appear, inline, on the right-hand side of the page.
You can also access context-relevant help pages from the Help menu, a the top-right of the Plesk Panel header.