For use in a multi buildpack:

1. Configure heroku for multiple build packs.

heroku config:set BUILDPACK_URL=https://github.com/polarsteps/heroku-diff-pdf-buildpack.git

2. Add pdftk to .buildpacks file

echo "https://github.com/polarsteps/heroku-diff-pdf-buildpack.git" >> .buildpacks

The minimum possible list of packages required to compile diff-pdf on Ubuntu are mentioned upstream.

Compile the binary using heroku/heroku:22 docker image. Pull the diff-pdf binary and move it to the binaries/ directory.

Once the app is deployed using the diff-pdf buildpack, there might still be some libraries that don't get installed. Find them out with ldd /path/to/diff-pdf on a running Heroku instance, and add these packages to the PACKAGES list.