Two things come to mind: Notepad ++ and Visual Studio (Express editions should also do the trick).
Notepad ++ is a generic text editor with highlighting for the main text, but it has some plugins that improve its XML processing capabilities. Reindenting is built into TextFx. Because its simple, large files are not a problem for him.
Visual Studio uses XML files in many places, so it has a built-in editor. It supports editing circuits (both with a graphical interface and with text mode), automatic formatting, syntax highlighting, even limited intellisense when there is a circuit. The speed is also pretty decent, there is no slowdown even on large documents.
Added: I'm not sure what a "beautiful style" is, but if it means reformatting a poorly formatted XML document, some search engines will find this tool . This is a command line utility, but you can add a command to it in the NP ++ menu. It is also available as a library, so you can develop an NP ++ plugin with it if you really should.
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