At Okular's webpage I see that its latest version is 1.2, released in August 2017. In my Ubuntu 16.04 I have Okluar, which says in its About section that it is version 0.25.0. If I try to update Okular in command line, it says okular is already the newest version (4:16.04.3-0ubuntu1~ubuntu16.04~ppa62). At Okular's webiste they just say "For GNU/Linux and the BSDs use the package manager to find the Okular package", which I did. Is this because ubuntu repositories still don't have updated version of Okular?
Canonical support the version originally installed with 16.04 LTS for five years; which means any security updates are backported to the supported version.
If however you need later features not in the installed version (as seen on the website) you can always try installing a 'snap' version which is a recent alternative. I did a
sudo snap install okularand it reportedly installed version 16.12.3 (revision 3) from KDE, so just install the snap version if you want later.
(note: Okular is a KDE app; uses Qt libs so will use more memory than using evince in standard Ubuntu which uses GTK+. Not a problem is you've got decent memory; but worth considering if you're 2GB or less)
addendum: snap wasn't installed by default in 16.04; so you may need to install it to issue 'snap' commands. please refer How to install snappy packages in Ubuntu 16.04