As s_ha_dum suggested, this has been made presuming it's being run on a *nix operating system, and I don't plan to make it work on Windows myself. However, if you get it to work I'm happy to include your changes in the plugin and list you as an author.
All the code for
pdftk should be in the file
Find out which errors the plugin is producing by adding the following to the site's
Errors should then appear in
wp-content/debug.log. I'm happy to help if you don't undertand what an error means.