Open XML is an open ECMA 376 standard and is also approved as the ISO/IEC 29500 standard that defines a set of XML schemas for representing spreadsheets, charts, presentations, and word processing documents. Microsoft Office Word 2007, Excel 2007, and PowerPoint 2007 all use Open XML as the default file format.
OpenXML SDK is required for exporting Telerik reports in OpenXML formats as DOCX, XLSX and PPTX.
The Open XML file formats are useful for developers because they use an open standard and are based on well-known technologies: ZIP and XML.
In case you use greater version of the OpenXML SDK, you need to add bindingRedirect in the application configuration file for the DocumentFormat.OpenXml.dll assembly.