How to attach files on android to send by email?

You have to get the URI of the file, so that:

 Uri uri = Uri.fromFile(new File(Environment.getExternalStorageDirectory().getAbsolutePath() + "/CALC/REPORTS/",pdfname ));

An example:

String[] mailto = {""};
Uri uri = Uri.fromFile(new File(Environment.getExternalStorageDirectory().getAbsolutePath() + "/CALC/REPORTS/",pdfname ));
Intent emailIntent = new Intent(Intent.ACTION_SEND);
emailIntent.putExtra(Intent.EXTRA_EMAIL, mailto);
emailIntent.putExtra(Intent.EXTRA_SUBJECT, "Calc PDF Report");
emailIntent.putExtra(android.content.Intent.EXTRA_TEXT,"Hi PDF is attached in this mail. ");
emailIntent.setType("application/pdf");
emailIntent.putExtra(Intent.EXTRA_STREAM, uri);
startActivity(Intent.createChooser(emailIntent, "Send email using:"));

Update: Open folder:

private void showFile(Uri fileUri){
    Intent intent = new Intent();
    intent.setAction(Intent.ACTION_VIEW);
    intent.setDataAndType(fileUri, "file/*");
    startActivity(intent);
}

Also:

startActivity(new Intent(Intent.ACTION_VIEW, Uri.parse("content://media/external/"))); /** Reemplazar con tu propio uri */

Attach several PDFs:

final Intent ei = new Intent(Intent.ACTION_SEND_MULTIPLE);
ei.setType("plain/text");
ei.putExtra(Intent.EXTRA_EMAIL, new String[] {"[email protected]"});
ei.putExtra(Intent.EXTRA_SUBJECT, "That one works");

And then you have to add the URIS:

ArrayList<Uri> uris = new ArrayList<Uri>();

ei.putParcelableArrayListExtra(Intent.EXTRA_STREAM, uris);
startActivityForResult(Intent.createChooser(ei, "Sending multiple attachment"), 12345);

Not only can you attach a .pdf, you can attach any type of files, in this case the type that would be determined by the MIME TYPE that for a .pdf file must be "application/pdf" .

Attaching a file when sending can be done through an Intent where you define the Uri resource if it is only one or a list of Uri 's to send multiple files:

        String archivoPDF = "archivo.pdf";
        //Obtiene la Uri del recurso.
        Uri uri = Uri.fromFile(new 
File(Environment.getExternalStorageDirectory().getAbsolutePath() + "/PDFS/", archivoPDF));
        //Crea intent para enviar el email.
        Intent i = new Intent(Intent.ACTION_SEND);
        i.setType("application/pdf");
        //Agrega email o emails de destinatario.
        i.putExtra(Intent.EXTRA_EMAIL, new String[] { "[email protected]" });
        i.putExtra(Intent.EXTRA_SUBJECT, "Envio de archivo PDF.");
        i.putExtra(Intent.EXTRA_TEXT, "Hola te envío un archivo PDF."); 
        i.putExtra(Intent.EXTRA_STREAM,  uri);
        startActivity(Intent.createChooser(i, "Enviar e-mail mediante:"));

Your question refers:

  

How to attach files in android studio to send by email?

If you want to attach several files, you must create a List of Uri 's which will contain the paths of the files.

  //Crear una Lista de Uris.
  List<Uri> listUris = new ArrayList<Uri>();
  //Agrega elementos (como ejemplo 3 elementos).
  lstUris.add(Uri.fromFile(new File(Environment.getExternalStorageDirectory().getAbsolutePath() + "/PDFS/", "archivo1.pdf")););
  lstUris.add(Uri.fromFile(new File(Environment.getExternalStorageDirectory().getAbsolutePath() + "/PDFS/", "archivo2.pdf")););
  lstUris.add(Uri.fromFile(new File(Environment.getExternalStorageDirectory().getAbsolutePath() + "/PDFS/", "archivo3.pdf")););

    //Crea intent para enviar el email.
    Intent i = new Intent(Intent.ACTION_SEND);
    i.setType("application/pdf");            
    //Agrega email o emails de destinatario.
    i.putExtra(Intent.EXTRA_EMAIL, new String[] { "[email protected]" });
    i.putExtra(Intent.EXTRA_SUBJECT, "Envio de archivos PDF.");
    i.putExtra(Intent.EXTRA_TEXT, "Hola te envío archivos PDF."); 
    //Define listado de Uris de cada archivo.
    i.putExtra(Intent.EXTRA_STREAM,  listUris);
    startActivity(Intent.createChooser(i, "Enviar e-mail mediante:"));