How to open and convert CHM documents?

You can convert them to PDF using the command line program chm2pdf (install chm2pdf here). Once installed you can run the command from a terminal like this:

chm2pdf --book in.chm out.pdf

In case you didn't know, there are several chm readers available - just search chm in the Software Centre.

You can also extract chm files to html using the command line tool 7-Zip (install p7zip-full here):

7z x file.chm

If you do not want to use PDF then I would suggest Epub, a fairly good, open e-book format, you can install a good reader for it called Calibre on Ubuntu, Calibre has a useful conversion facility that can import chm files and then convert them to other formats epub included. epubs can be easily read on most smart phones and tablets as well.

Calibre can be installed from the software center.

There is also KChmViewer, if you prefer KDE.