You can uninstall or removes an installed diffpdf package itself from Ubuntu 16.10 (Yakkety Yak) through the terminal,
$ sudo apt-get remove diffpdf
If you would like to remove diffpdf and it's dependent packages which are no longer needed from Ubuntu,
$ sudo apt-get remove --auto-remove diffpdf
If you use with purge options to diffpdf package all the configuration and dependent packages will be removed.
$ sudo apt-get purge diffpdf
If you use purge options along with auto remove, will be removed everything regarding the package, It's really useful when you want to reinstall again.
$ sudo apt-get purge --auto-remove diffpdf