... in case the online documentation goes offline (not sure if this was already asked) 

It is in the SONAR Local Documentation installer. The SONAR Reference Guide pdf was recently updated. The default path for the file is "C:\Program Files\Cakewalk\SONAR Platinum" (or Professional, Artist)

already have it ?!? sweet, thanks for the info, should have rtfm... :)

Note it has a download selection in the Sonar options in Command Center.

Well, there are a couple of steps actually that you have to take because Sonar always defaults to using the online help pages.
 
1.  On Cakewalk's Website go to your "My Account" > My Products > Sonar Platinum > Connected Products > SONAR Local Documentation and download the separate installer file "SONARHelp_Documentation_1.0.0.56" which is the latest version and likely the last version to be written! :-(
 
2.  Run the setup file and then ensure you follow these steps once it's installed; run Sonar and go to the "Help" Menu > Preferences > Advanced and select the tickbox "Always Use Local Help" or Sonar will still go off to the online help pages.
 
This is an important step if they ever turn off the online servers otherwise you won't find any help within Sonar works.
 
Hope that helps.

I suspect at this point, not a lot of money and time will be spent updating either website or local documentation files. I did find my local help so I'm glad the question for this thread was asked and I followed the instructions to verify I have it downloaded locally.