How to remove plastic film on window pane
Unless it peels off, which it apparently doesn't, it's probably an adhesive-backed clear coat application. You don't mention if the window is glass or acrylic (Plexiglass). Assuming it's glass you might be able to get it off with a safety razor scraper. Any adhesive could possibly be dissolved with finger nail polish remover or a commercial product such as Goof Off. 

If it's acrylic, any scraping or solvent application may damage the pane. In that instance, you may just want to replace it but if you're going to do that you might want to try scraping and solvent first. Nothing to lose.