as the answers reflect: markdown for simple stuff (sou can convert with pandoc) and libreoffice for the more complex stuff and sheets especially (its preinstalled with most linux distros nowadays). documents of formal nature that exceed ~10 pages might work best in latex.
Most people don't know this, but OpenOffice is pretty much dead. It hasn't been getting any real updates for quite a while. LibreOffice is pretty active and is the one you'd want to go with.