Install helpviewer.app by entering the following commands in the terminal:
sudo apt update sudo apt install helpviewer.app
Description:
Online help viewer for GNUstep programs
HelpViewer is an online help viewer for GNUstep programs. It can render files in the XLP format, although this format has been deprecated and only several packages still use it.
Homepage: http://www.roard.com/helpviewer/
Version: 0.3-8build3
Section: universe/doc