For a long time the TextFX plugin did the job for the Notepad++ 32-bit version via the menu
TextFX > TextFX HTML Tidy > Tidy: Reindent XML.
In January 2012 Notepad++ v5.9.8 didn't include the plugin anymore, probably due to new support of Unicode, which the developer of TextFX didn't embrace soon enough, see N++ v5.9.8 changelog vs N++ v5.9.6 changelog and TextFX's Future
Later was introduced a 64-bit version of Notepad++ which didn't even include the Plugin Manager (because it was itself a plugin, developed by a third party developer).
Until recently. In April 2017, the developer of the NppPluginManager announced there was a build of a x64 version available:
Now the problems of either using an outdated 32-bit version of N++ with the TextFX plugin, or trying to install Tidy2 (or others) to more recent versions of N++ (but still x32!!), are gone!