Open new Numbers document in older Numbers
This is a hack, but assuming that your document won't be damaged by undergoing some slight changes, here is what I would do.


Logon to www.iCloud.com, and open Numbers. If it isn't there, go to beta.iCloud.com.
Upload the new Numbers document to iCloud and then open the document in your browser.
Select Download a Copy from the settings menu (as seen in the screenshot below) and choose to save it as a Word document.
Open the Word document in Numbers and save/export in whatever format you wish.




Alternate Approach:

You could, depending on the nature of the document, send it to a friend who has Yosemite and ask them to export it to the Numbers '09 format.

Other Options:


If I were you, and did want to mess with importing/export I'd probably try CloudConvert.com.
This article claims you may be able to revert the document, although the fact that you can't open it in the first place would seem to negate that option.