ChmSee is a Compiled HTML Help (CHM) file viewer for Unix/Linux.
It is based on CHMLIB and use GTK+ as its frontend toolkit. Because of using gecko HTML rendering engine, ChmSee can support rich features of modern HTML page, specially CSS.
If you need to open a chm file in Ubuntu,just use below command to install it since it already been included in the repositories.
sudo apt-get install chmsee
Below is a screenshot of ChmSee with file opened