This has not been published for inclusion in the list of extensions you can install from within CiviCRM, so you're going to need to do it manually.
CiviCase component enabled.
Installation from zipped release file¶
Visit https://github.com/artfulrobot/pelf/releases to see the latest release, and download the .zip (or .tar.gz) file from there.
Unzip that into your extensions directory. Once it's there you should be able to install from the admin interface, after clicking the Refresh button on the extensions page.
Installation from cloned Git repository (developers)¶
Clone https://github.com/artfulrobot/pelf into your extensions dir. Then
install via the UI, or use
cv en pelf.