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 “@”.
@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.