On https://github.com/jgm/pandoc/releases, you'll see a list of Pandoc releases.
Click one of them to see the available files in that release.
You can right click a given link to copy its URL to put that into a script.
The gist of the following script (which you can put in your postInstall is to
cURLa modern pandoc release, move the binaries to where they'll be found, and deletes everything else:
#!/usr/bin/env bash
set -e
curl -L https://github.com/jgm/pandoc/releases/download/2.8.0.1/pandoc-2.8.0.1-linux-amd64.tar.gz | tar xz
mv ./pandoc-2.8.0.1/bin/pandoc /usr/bin
mv ./pandoc-2.8.0.1/bin/pandoc-citeproc /usr/bin
rm -rf ./pandoc-2.8.0.1/