Для тех, кому нужен ответ, вот он ....
Application xlApp = new Application();
xlApp.DisplayAlerts = false;
Workbook book = xlApp.Workbooks.Open(@"C:\test5.xls", 2, false);
Worksheet sheet = book.Worksheets[1];
sheet.Shapes.AddOLEObject(Filename: @"C:\test.pdf", Height: 400, Width: 400);
book.Save();
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