This is bad practise. It works but it is better to install using that
repositories for many reasons, but having updates is the biggest. It also is
easier to uninstall. This also encourages Windows users to search for
applications on the internet as they do in Windows and it can lead to mega
problems. Many Linux veterans even cringe at the thought of PPAs, and would
never think to do what is being advised.

Skype is proprietary although gratis which is why it is not found in the
regular repos. You can get it by simply enabling the partner repositories
though.