An iWork '09 file created from Pages, Numbers, or Keynote actually lives a secret life as a ZIP archive. This trick isn't a new one; the most common example of archive trickery by Apple is probably the iPod/iPhone software bundle which uses ZIP as a container format. Previous iWork versions actually created folder-like bundle files by default, which made them tricky to upload to cloud storage or email to collaborators.
In the case of the new iWork files, changing the file extension to .zip and expanding the archive reveals the as-expected XML document and plist files (for the document and its formatting) as well as a little PDF surprise -- a preview version of the file. What this means for you is that you can send an iWork file to a friend or co-worker and regardless of their operating system choice or installed software base, they can "view" your document by unzipping it and opening the PDF.
On Windows, you would simply change the extension to .zip and open with your favorite un-archiving utility. Given the kludgy-ness of this process, it doesn't surprise me that Apple isn't promoting it. However, it would be interesting to see the folks in Cupertino release an iWork viewer for the Windows users in our lives (other than the extant iwork.com sharing service).
[Via Download Squad]