This PPA will have what the Document Foundation calls “LibreOffice fresh”, the latest release of the newest series (but no alpha/beta releases).

There is a PPA dedicated to specific LibreOffice major series which support a range of older Ubuntu releases too:
https://launchpad.net/~libreoffice/+archive/libreoffice-6-1 (“Fresh”)
https://launchpad.net/~libreoffice/+archive/libreoffice-6-0 (“Still”)
 (testbed for 18.04 LTS SRUs)
https://launchpad.net/~libreoffice/+archive/libreoffice-5-4 (EOL)
https://launchpad.net/~libreoffice/+archive/libreoffice-5-1 (EOL)
 (testbed for 16.04 LTS SRUs, EOL upstream)
https://launchpad.net/~libreoffice/+archive/libreoffice-4-2 (EOL)
 (testbed for 14.04 LTS SRUs, EOL upstream)

Alpha and beta releases of a new major releases and the first release candidate of minor updates can be found at:
https://launchpad.net/~libreoffice/+archive/libreoffice-prereleases

So much for other ppas.

Please file bugs for these packages on launchpad as described in:
https://lists.launchpad.net/libreoffice/msg00072.html

This PPA might contain the release candidate that is assumed to become the final release even before it is declared so by the Document Foundation (e.g. usually release candidate 2 for minor updates).

Most of the packages in this PPA have only experienced minor testing — in fact it is the place to enable a wider audience to test packages before they are published into the distro proper. In general, this PPA is _not_ for the average user to install without a closer look (if it would be, its packages would be in the main repositories). OTOH, it is _way_ _better_ to use packages from this PPA than using the *.deb files that The Document Foundation provides upstream, which are intentionally build against a very old baseline for maximum compatibility. So, _if_ you want to be on the bleeding edge, do it here, not with upstream *.debs.

In general, users are advised to take a look at the changelog for the details about a package. If there is a specific bug that is intended to be addressed by an update released into the PPA, you are encouraged to test, if the update solves that problem. Packages published after the distro release are mostly such specific fixes. Critical fixes will be SRUed into the main repositories after testing anyway (later, with more testing).

To return to the LibreOffice version from the main archive, use ppa-purge. see: http://www.webupd8.org/2009/12/remove-ppa-repositories-via-command.html for details
More info: https://launchpad.net/~libreoffice/+archive/ubuntu/ppa
Press [ENTER] to continue or ctrl-c to cancel adding it

gpg: keybox ‘/tmp/tmpf5yudwep/pubring.gpg’ created
gpg: key 83FBA1751378B444: 2 signatures not checked due to missing keys
gpg: /tmp/tmpf5yudwep/trustdb.gpg: trustdb created
gpg: key 83FBA1751378B444: public key “Launchpad PPA for LibreOffice Packaging” imported
gpg: no ultimately trusted keys found
gpg: Total number processed: 1
gpg: imported: 1
gpg: no valid OpenPGP data found.
Exception in thread Thread-1:
Traceback (most recent call last):
File “/usr/lib/python3.5/threading.py”, line 914, in _bootstrap_inner
self.run()
File “/usr/lib/python3.5/threading.py”, line 862, in run
self._target(*self._args, **self._kwargs)
File “/usr/lib/python3/dist-packages/softwareproperties/SoftwareProperties.py”, line 688, in addkey_func
func(**kwargs)
File “/usr/lib/python3/dist-packages/softwareproperties/ppa.py”, line 386, in add_key
return apsk.add_ppa_signing_key()
File “/usr/lib/python3/dist-packages/softwareproperties/ppa.py”, line 273, in add_ppa_signing_key
cleanup(tmp_keyring_dir)
File “/usr/lib/python3/dist-packages/softwareproperties/ppa.py”, line 234, in cleanup
shutil.rmtree(tmp_keyring_dir)
File “/usr/lib/python3.5/shutil.py”, line 480, in rmtree
_rmtree_safe_fd(fd, path, onerror)
File “/usr/lib/python3.5/shutil.py”, line 438, in _rmtree_safe_fd
onerror(os.unlink, fullname, sys.exc_info())
File “/usr/lib/python3.5/shutil.py”, line 436, in _rmtree_safe_fd
os.unlink(name, dir_fd=topfd)
FileNotFoundError: [Errno 2] No such file or directory: ‘S.gpg-agent.extra’

For Ubuntu 18.04 this will do the trick:

sudo add-apt-repository ‘deb https://www.collaboraoffice.com/repos/CollaboraOnline/CODE-ubuntu1804 ./’