How do I open an apple keynote file in windows?

The following websites can convert the .key to PDF online:

• https://cloudconvert.com/key-to-pdf (tested on 2019-02-01)
• http://www.zamzar.com (tested on 2013-08-15)

Also, the .key is a zip archive: if you unzip you may be able to see a thumbnail for each slide.

There is now a keynote viewer for the browser using icloud. If you have a Apple device, you can make an icloud account, and upload the file to your account. then go to icloud's website and play it on any browser.