The original pugixml software had the following license text:

Original Text

The pugixml library is distributed under the MIT license:

     Copyright (c) 2006-2010 Arseny Kapoulkine

     Permission is hereby granted, free of charge, to any person obtaining a copy of this software
     and associated documentation files (the "Software"), to deal in the Software without restriction,
     including without limitation the rights to use, copy, modify, merge, publish, distribute,
     sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is
     furnished to do so, subject to the following conditions:

     The above copyright notice and this permission notice shall be included in all copies or
     substantial portions of the Software.

     THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING
     BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
     NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM,
     DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
     OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.

This means that you can freely use pugixml in your applications, both open-source and proprietary.
If you use pugixml in a product, it is sufficient to add an acknowledgment like this to the product
distribution:

     This software is based on pugixml library (http://pugixml.org).
     pugixml is Copyright (C) 2006-2010 Arseny Kapoulkine.

The documentation was copied with permission per the copyright notice near the bottom of Table of contents and several other pages as follows:

Copyright © 2010 Arseny Kapoulkine
Distributed under the MIT License