==Pi Based Announcement Digital Sign==
If you want to autorun a command on a Raspberry Pi within the LXDE GUI at startup and login edit

 sudo nano /etc/xdg/lxsession/LXDE-pi/autostart

and add your command at the end after a "@". 

I added 

@soffice -show /home/pi/Documents/announcements.odp

Within LibreOffice Impress Slide Transisions (tool bar on right), Advance Slide, Automatically and set to number of seconds then "Apply Transition to All Slides". Then Slide Show > Slide Show Setting > Loop and repeat after 0 seconds. Save and reboot.