From mediawiki.org

Electron is a PDF rendering service that was used on Wikimedia sites for single-page PDF rendering (the "Download as PDF" option). In 2019 June, it was replaced by Proton.

It uses the Electron browser control tool and the Chromium rendering engine. It lives in the mediawiki/services/electron-render repo.

See the README and the associated ElectronPdfService extension for more info.