What libopc is:
The Open Packaging Convention (OPC) is part of the ISO/IEC 29500 specification which standardizes word-processing, spreadsheet and presentation documents. A prominent implementer of the new standard is Microsoft Office Version 2007 and beyond. Every .docx, .xslx, .pptx document is an OPC container. Libopc has functions to read/write OPC container.
Another part of ISO/IEC 29500 is Markup Compatibility and Extensibility (MCE). Libopc also has features to parse and generate XML markup compatible with MCE.