Numbers is a Apple only spreadsheet program (as well as Pages and Keynote), so it should have a ability to export as Excel or Word compatible file format, which is then readable by Window Excel/Word or the free cross platform LibreOffice.
In fact, if your going to be working with Windows users, you should be using the free LibreOffice suite as there is a free version for Windows, Linux and OS X, so the files are cross platfrom compatible. It also reads Microsoft Excel, Word etc., formats too.
10.8 users will likey need to head to System Preferences > Security > General and set the downloads to "Anywhere" to accept software from the web first before downloading. (Gatekeeper)
Is there any program or sth. on pages to convert the data?
If you just want to send the data as a image to look at, without them being able to mess with it or edit it, then you can open the file in the program and select File > Print.
On the lower left corner is PDF... click it.
Save the file as a PDF onto your desktop and then from the Mail program attach it or upload it to your email to them.
This allows them to see it, but they don't need the program or a compatible one to edit the real file.
As long as it's not too huge, you can send attachments, but if they reach a certain size the ISP will not upload it or email service will not send it.
You can send several smaller files over more emails, or use a online Dropbox type service and then send them the download link in a email for larger sized files.