Generate a spreadsheet file on iOS that is readable both by Excel and Numbers

I need to generate a spreadsheet file on iOS that is readable both by Excel and Numbers. CSV file is not sufficient as it does not allow multiple sheets and/or formatting.

Which format would work the best? And is there a library/frame开发者_如何学Cwork that I could use to generate a file in that format?

Create a Google docs spreadsheet the Google Data APIs Objective-C Client Library. Then you can import that into Excel/Numbers.