How to download a file which I want to use processbar for that?

The Dropbox.NET SDK doesn't currently offer a way to get the progress of a download, but I'll be sure to pass this along as a feature request. 

It also doesn't offer a way to download folders in bulk, so you'll need to iterate through to download each desired file.

Actually, for reference, you can track progress if you use GetContentAsStreamAsync. E.g., you can do something like:

 var response = await client.Files.DownloadAsync(path);
 ulong fileSize = response.Response.Size;
 const int bufferSize = 1024 * 1024;
 
 var buffer = new byte[bufferSize];
 
 using (var stream = await response.GetContentAsStreamAsync())
 {
     using (var file = new FileStream("Test", FileMode.OpenOrCreate))
     {
         var length = stream.Read(buffer, 0, bufferSize);
 
         while (length > 0)
         {
             file.Write(buffer, 0, length);
             var percentage = 100 * (ulong)file.Length / fileSize;
             // Update progress bar with the percentage.
             // progressBar.Value = (int)percentage
             Console.WriteLine(percentage);
 
             length = stream.Read(buffer, 0, bufferSize);
         }
     }
 }

I have tried it but I don't know where is file be saved?

public async void Download(string token, string path)
{
using (var dbx = new DropboxClient(token))
{
var download = await dbx.Files.DownloadAsync(path);
}
}

In your sample code, the file isn't saved anywhere. The data is just available via the download object.

In my sample, the file is saved to the path "Test", but if you use that you should of course change that as desired.