tar.xz extracting error "xz: (stdin): File format not recognized"

file.tar.xz: HTML document, UTF-8 Unicode text, with very long lines

indicates that I downloaded a website. That's why it won't extract.

Please do not download files from a random site. The contents of that file could have been altered to mess up you system.

The correct place to download from is the debian wiki page and you download it from their tracker site and here is the direct download. It will download pepperflashplugin-nonfree_1.8.1+deb8u1.tar.xz and after downloading you can use your tar command to extract it.

To install though I would advice to use the normal Ubuntu installation. Enable "Canonical Partners" in Software & Updates and then run ...

sudo apt update
sudo apt install adobe-flashplugin

This will install flash and the pepper flash plugin.