I would recommend Notepad++ to do all sorts of heavy duty file editing. The current version, 7.0, is a new release and when you go for the 64bit flavor, and compatible plugins/add-ons are few, currently. Many of the plugins which do so much of the formatting for various flavors of files have not yet been updated, but still work well with the prior versions.
I have even Notepad++ as the editor for multiple development environments. Versions prior to 7.0 have dozens of plugins, a nice selection of which can indent/re-indent/tab convert/restructure source code and other formatted text, then even pass you edited file to a CL compiler.
Version 7 has a convert tab to space right out of the box, which may be sufficient for your needs. The add-on developers are slowly getting the 64bit versions of their stuff rolled out, but it always seems to take time for what you want to be released. :-(