The SONAR X2 Reference Guides and R-MIX SONAR documentation are available in the links below:

SONAR X2 Reference Guide (English) (Approx. 35 MB)
SONAR X2 Reference Guide (French) (Approx. 35 MB)
SONAR X2 Reference Guide (German) (Approx. 35 MB)
R-MIX SONAR Documentation (Approx. 3.6 MB)
To read, unzip first. Afterwards right-click on .chm, select properties, and click [Unblock] then [Apply].