sudo add-apt-repository ppa:libreoffice/libreoffice-4-2
sudo apt install libreoffice
sudo apt-get install ffmpeg
sudo apt-get install unoconv
sudo apt-get install ghostscript
sudo apt-get install php-imagick
sudo apt-get install imagemagick

See also Ubuntu Install

Admin > File Galleries > General Settings

Settings for Media Alchemyst (with default):

ffmpeg path - /usr/bin/ffmpeg
ffprobe path - /usr/bin/ffprobe
unoconv path - /usr/bin/unoconv
ghostscript path - /usr/bin/gs
Alchemy Image library - ImageMagick

The Imagemagick security policy prevents PDF to image conversion.