The easiest way is to email the document to yourself. You could also put it into dropbox (or skydrive, box.net or any of the other online storage serices), then open the dropbox app on your ipad, or sync the documents through itunes.
If you do it via email, then open the attachment, it will let you view it with in the email. In the upper right corner, there is an arrow icon coming out of a box, click on that & it will offer you apps you can open the spreadsheet in. Just choose Numbers.