Just checked: In Excel it works as expected. But PowerPoint seems to have a problem.
[TestMethod]
public void SaveExcelAsPdf()
{
NetOffice.ExcelApi.Application app = new NetOffice.ExcelApi.Application();
app.Workbooks.Add();
app.ActiveWorkbook.Worksheets.Add();
((NetOffice.ExcelApi.Worksheet)app.ActiveSheet).Cells[1, 1].Value = "hello!!!";
((NetOffice.ExcelApi.Worksheet)app.ActiveSheet).ExportAsFixedFormat(NetOffice.ExcelApi.Enums.XlFixedFormatType.xlTypePDF, @"C:\Mappe1.pdf");
}