Microsoft.Office.Interop.Word is the primary library used to automate and manipulate Word documents (e.g., editing headers, converting to PDF), whereas Microsoft.Office.Core provides shared Office-related interfaces (like ribbon customization) but cannot handle document-specific tasks. While Interop.Word works well on desktop systems, it often crashes or behaves unpredictably on servers like Windows Server 2008 R2 because Microsoft does not support Office automation in server-side environments. For stable server-side document processing, consider alternatives like Open XML SDK (for document manipulation) and LibreOffice CLI, Aspose.Words, or Syncfusion (for PDF conversion without needing Word installed).