System.UnauthorizedAccessException when writing to external storage on Android

I am using following code to download a pdf file from web-

public void DownloadFile(string fileName){

        var documentsPath = Environment.GetFolderPath (Environment.SpecialFolder.Personal);
        string filePath = Path.Combine (documentsPath, fileName);

        using (var writer = File.OpenWrite(filePath))
        {
            var restClient = new RestClient(RecepieUrl);
            var request = new RestRequest(Method.GET);
            request.ResponseWriter = (responseStream) => responseStream.CopyTo(writer);
            var responseData = restClient.DownloadData(request);
        }
    }

I then try reading the file using depenedency service as follows-

public void OpenFile (string fileName)
{
//get the intent
Intent intent = new Intent(Intent.ActionView);

        var documentsPath = Environment.GetFolderPath (Environment.SpecialFolder.Personal);
        string filePath = Path.Combine (documentsPath, fileName);

        var bytes = File.ReadAllBytes(filePath);

        //Copy the private file's data to the EXTERNAL PUBLIC location
        var externalPath = global::Android.OS.Environment.ExternalStorageDirectory.Path + fileName;


        File.WriteAllBytes(externalPath, bytes);



        intent.SetDataAndType(global::Android.Net.Uri.Parse(externalPath), "application/pdf");
        intent.SetFlags(ActivityFlags.NewTask);

        try {
            var context = Forms.Context;
            context.StartActivity(intent);
        } catch (Exception e) {
            string s = e.Message;
        }
    }

But I keep getting following exception-

System.UnauthorizedAccessException: Access to the path "/storage/emulated/074.pdf" is denied.
at at System.IO.FileStream..ctor (string,System.IO.FileMode,System.IO.FileAccess,System.IO.FileShare,int,bool,System.IO.FileOptions)
at at System.IO.FileStream..ctor (string,System.IO.FileMode,System.IO.FileAccess,System.IO.FileShare,int)
at at System.IO.File.Create (string,int)
at at System.IO.File.Create (string)
at at System.IO.File.WriteAllBytes (string,byte[])

I have assigned WRITE_EXTERNAL_STORAGE in my manifest file.

Thanks