Plugin - Aspose

if (ext == "doc" || ext == "docx")
{
    Aspose.Words.Document doc = new Aspose.Words.Document(templateFile);
    doc.Save(generateFilePath, Aspose.Words.SaveFormat.Html);
}
else if (ext == "xls" || ext == "xlsx")
{
    Workbook workbook = new Workbook(templateFile);
    workbook.Save(generateFilePath, SaveFormat.Html);
}
else if (ext == "ppt" || ext == "pptx")
{
    templateFile = templateFile.Replace("/", "\\");
    PresentationEx pres = new PresentationEx(templateFile);
    pres.Save(generateFilePath, Aspose.Slides.Export.SaveFormat.Html);
}