as an attachment to a PDF file in gmail using an intent in android studio 3.1

Hi, I have this problem and I hope you can guide me.

I'm programming an application and I need to send a PDF file, I've already made the program and it's like this:

import android.content.Intent;
import android.net.Uri;
import android.os.Environment;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.view.View;
import android.widget.Button;
import android.widget.EditText;    
import java.io.File;

public class MainActivity extends AppCompatActivity {


@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);

    ((Button) findViewById(R.id.ButtonSendFeedback)).setOnClickListener(new View.OnClickListener() {
        public void onClick(View v) {
            String to = ((EditText) findViewById(R.id.txtTo)).getText().toString();
            String sub = ((EditText) findViewById(R.id.txtSubject)).getText().toString();
            String mess = ((EditText) findViewById(R.id.txtMessage)).getText().toString();
            String serv = ((EditText) findViewById(R.id.txtserv)).getText().toString();
            String cuerpo = ((EditText) findViewById(R.id.txtBody)).getText().toString();
            String archivopdf = "Download/Ordendeservicio.pdf";


            Uri uri = Uri.fromFile(new File(Environment.getDownloadCacheDirectory().getAbsolutePath(), "file://" + archivopdf ));

            Intent mail = new Intent(Intent.ACTION_SEND);
            mail.putExtra(Intent.EXTRA_EMAIL, new String[]{to});
            mail.putExtra(Intent.EXTRA_SUBJECT, sub);
            mail.putExtra(Intent.EXTRA_TEXT, "orden de servicio" );
            mail.putExtra(Intent.EXTRA_STREAM, uri);
            mail.setType("message/rfc822");
            startActivity(Intent.createChooser(mail, "Send email via:"));
        }
    });
}

}

The code will open the gmail apk, but when it does, it tells me that the file can not be attached.

What do you think is the error that I have or that I have not configured correctly? Thank you in advance for any comments.