An XSD file stores its content as plain text in XML format, which means that the file can be opened and viewed with any text editor such as Notepad or Notepad++.
XSD is short for XML Schema Defition, and it is a specification which formally describes the elements and their relationships in an Extensible Mark-Up Language.
The file is used to define what elements and attributes may appear in an XML document. In this case this is a formal description of the iXBRL elements, and as such, your auditor doesn't really need to open it.