It's used to view a PDF through the browser, without the need to download the file via the Plugin MediaPlayer.
In order to make it work you first need to install npm-asset/pdfjs-dist-viewer-min from Control Pannels >package menu.
So, to active the feature, go to Control Control pannels >File Galleries >Enhancements, or you can search for fgal_pdfjs_feature or pdfjs.