7zip (*.7z) is one of the commonly used compression file format in the universe, not as popular as zip and tar, but some people use 7zip to compress files.
By default 7zip is not supported by lubuntu, that means normally you can't work with *.7z files, you simply can't open it on lubuntu.
We need to do something in order to make lubuntu to support 7zip compression format, we need to install package called 'p7zip-full'.
Okay, go ahead install the package, open your terminal/console/command line (press CTRL + ALT + T) and then run this command to enable 7zip support on lubuntu.
sudo apt-get install p7zip-full
Once installed, you can try to open *.7z files from the file manager which in this case is PCManFM, double click the file and it should work, now lubuntu can handle *.7z files.
Extracting 7zip from command line
7z x [name-of-the-7zip-file]
7z x mydata.7z
7z x secret.7z
7z x naked-woman.7z
What if p7zip still can't open/extract my 7zip files
Well, for some reasons if you still can't open or extract your 7zip (*.7z) files, there is an alternative way to open 7zip files.
There is a program called 'dtrx' which support many compression file format including 7zip, so basically you can try this if p7zip doesn't work for you.
sudo apt-get install dtrx
To use dtrx, simply run dtrx followed by the name of the compressed file, which could be anything from zip, tar, 7zip, or even rar.
~ don't worry be happy ~