You don't need a PPA, gscan2pdf is already available in 4.x
Uninstall the program, remove the PPA as per the Help Manual instructions.
Then:
apt policy gscan2pdf
should return something like:
gscan2pdf:
Installed: (none)
Candidate: 2.1.0-1
Version table:
2.1.0-1 500
500 http://us.archive.ubuntu.com/ubuntu bionic/universe amd64 Packages
500 http://us.archive.ubuntu.com/ubuntu bionic/universe i386 Packages
if the version info is the same as above, then:
sudo apt-get install gscan2pdf
and in future, consult the Forums before adding PPA's, they are complicated. Cheers