PDI files were used in older versions of Microsoft PowerPoint for various import/export operations done between MS Office programs (typically from PowerPoint to Word and back) and these files might not work in latest versions of PowerPoint.
If you truly need to export these .pdi files, it might be best to get access to old version of PowerPoint and import the .pdi files so you can export the presentation to PDF format. It is unlikely any other way how to achieve pdi to pdf conversion exists.
Updated: September 28, 2023