Updating ppa's for libreoffice gets:
sudo apt-add-repository ppa:libreoffice/libreoffice-5-0
...
Press [ENTER] to continue or ctrl-c to cancel adding it
...
gpg: key 1378B444: public key "Launchpad PPA for LibreOffice Packaging" imported
gpg: no ultimately trusted keys found
gpg: Total number processed: 1
gpg: imported: 1 (RSA: 1)
OK
How then might libreoffice ppa not be trusted?