Answer:
Adobe© Reader must be installed in order to use the Help function in our programs, including the link to the corresponding manual chapter. Links to other PDF programs are currently not possible.
The PDF files are located in the program directory of the application, in the "Help" folder. There, you can open them with any PDF software. For RSTAB 8.18, the path of the default installation is as follows: C:\Programme\Dlubal\RSTAB 8.18\Help