Brenden is correct it is browser and machine dependent.
You can also get third party controls that will allow you to edit .doc, docx, xls, etc. These I use in my .aspx pages. In Kentico I create a new webpart .ascx with the controls and then register it in Kentico and use it wherever I need to.
The ones that I currently use, which work flawless with Kentico are
Devexpress, (Word, Excel, rtf, csv and pdf)
ComponentOne (Spread for ASP.net, for xls, xlsx.)
SpreadSheetGear (full excel and editable in app reporting.)