جای image فایل pdf را قرار دهید

 using (var image=System.IO.File.Open(@"C:\Users\Madaeny\Desktop\New folder\Iman.jpg",FileMode.Open))
                            {
                                var reg = new SendPhoto(up.Message.Chat.Id, new FileToSend(image,"iman.jpg"));
                                await bot.MakeRequestAsync(reg);
                                
                            }