you already have office365 on the web (is supposed that it works on your web browser).
Also Microsoft haven't really focused on creating Apps... There is only 7 apps created by Microsoft on the App Store. (Probably they are focusing into their own system).
There is a couple of Alternative Apps that allows you to open and edit Office documents, like word or excel. The best ones are the paid apps anyway. The free apps that open office documents do not allow you to edit and save a document.
If you really want Office in your Ipad, better get a remote desktop app and do a RDP connection to whatever computer you use with MS Office.