Is there a free Office App for the iOS to read Word, Excel, PowerPoint & OneNote files?
These can be displayed by IOS with no software involved, other than OneNote.  The trick is getting the files onto the device.  Email is one way; you can view these files, other than OneNote, as attachments, using the ioS Mail app.

Another alternative is to use a sync service client (e.g. dropbox) to access the files.