Install Okular

 $ sudo apt install okular
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Reading package lists... Done
Building dependency tree       
Reading state information... Done
The following package was automatically installed and is no longer required:
  linux-hwe-5.15-headers-5.15.0-76
Use 'sudo apt autoremove' to remove it.
The following additional packages will be installed:
  libkf5js5 libkf5jsapi5 libkf5kexiv2-15.0.0 libkf5khtml-bin libkf5khtml-data
  libkf5khtml5 libkf5threadweaver5 libmarkdown2 libokular5core9
  libqmobipocket2 okular-extra-backends qtspeech5-flite-plugin
The following NEW packages will be installed:
  libkf5js5 libkf5jsapi5 libkf5kexiv2-15.0.0 libkf5khtml-bin libkf5khtml-data
  libkf5khtml5 libkf5threadweaver5 libmarkdown2 libokular5core9
  libqmobipocket2 okular okular-extra-backends qtspeech5-flite-plugin
0 upgraded, 13 newly installed, 0 to remove and 0 not upgraded.
Need to get 9.032 kB of archives.
After this operation, 32,7 MB of additional disk space will be used.
Do you want to continue? [Y/n] y
Get:1 http://id.archive.ubuntu.com/ubuntu focal/universe amd64 libkf5js5 amd64 5.68.0-0ubuntu1 [266 kB]
Get:2 http://id.archive.ubuntu.com/ubuntu focal/universe amd64 libkf5jsapi5 amd64 5.68.0-0ubuntu1 [19,9 kB]
Get:3 http://id.archive.ubuntu.com/ubuntu focal/universe amd64 libkf5kexiv2-15.0.0 amd64 19.12.3-0ubuntu1 [136 kB]
Get:4 http://id.archive.ubuntu.com/ubuntu focal/universe amd64 libkf5khtml-data all 5.68.0-0ubuntu1 [508 kB]
Get:5 http://id.archive.ubuntu.com/ubuntu focal/universe amd64 libkf5khtml5 amd64 5.68.0-0ubuntu1 [2.003 kB]
Get:6 http://id.archive.ubuntu.com/ubuntu focal/universe amd64 libkf5khtml-bin amd64 5.68.0-0ubuntu1 [108 kB]
Get:7 http://id.archive.ubuntu.com/ubuntu focal/universe amd64 libkf5threadweaver5 amd64 5.68.0-0ubuntu1 [51,9 kB]
Get:8 http://id.archive.ubuntu.com/ubuntu focal/universe amd64 libmarkdown2 amd64 2.2.6-1 [35,7 kB]
Get:9 http://id.archive.ubuntu.com/ubuntu focal/universe amd64 libokular5core9 amd64 4:19.12.3-2ubuntu1 [323 kB]
Get:10 http://id.archive.ubuntu.com/ubuntu focal/universe amd64 libqmobipocket2 amd64 4:19.12.3-0ubuntu1 [16,5 kB]
Get:11 http://id.archive.ubuntu.com/ubuntu focal/universe amd64 qtspeech5-flite-plugin amd64 5.12.8-0ubuntu1 [23,1 kB]
Get:12 http://id.archive.ubuntu.com/ubuntu focal/universe amd64 okular amd64 4:19.12.3-2ubuntu1 [5.358 kB]
Get:13 http://id.archive.ubuntu.com/ubuntu focal/universe amd64 okular-extra-backends amd64 4:19.12.3-2ubuntu1 [182 kB]
Fetched 9.032 kB in 6s (1.429 kB/s)                                            
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Setting up okular (4:19.12.3-2ubuntu1) ...
Processing triggers for desktop-file-utils (0.24-1ubuntu3) ...
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Processing triggers for libc-bin (2.31-0ubuntu9.9) ...
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Processing triggers for bamfdaemon (0.5.3+18.04.20180207.2-0ubuntu2) ...
Rebuilding /usr/share/applications/bamf-2.index...