pdftk get first in list as most recommended way when searching for editing PDF metadata in Ubuntu. But
pdftk is no longer present in Ubuntu 18.04 repositories.
Found a different tool named PDFMtEd, which is actually a GUI front-end for ExifTool.
Here is how installed and used it successfully.
First install its dependencies:
sudo apt install libimage-exiftool-perl yad qpdf git
Then clone Git repository of PDFMtEd.
git clone https://github.com/glutanimate/PDFMtEd
Now got inside to the repo directory:
You may install it globally with command:
But it did not work for me and gave some error. However I could run it directory from repo directory:
Of course you can symlink to somewhere to make it a global command.