Adding an attachment to an email is pretty easy; you can use a convenience method to attach a file, or you can build it yourself.

Convenience method:

<?php
$file_path = '/path/to/file.pdf';
$mime_type = 'application/pdf';

// this will automatically set the attachment name
// to the basename() of the attached file
$mail = Solar::factory('Solar_Mail_Message');
$mail->attachFile($file_name, $mime_type);
?>

Or you can build and add a MIME part yourself.

<?php
$file_path = '/path/to/file.pdf';
$mime_type = 'application/pdf';

$part = Solar::factory('Solar_Mail_Message_Part');
$part->setDisposition('attachment');
$part->setFilename(basename($file_name));
$part->setContent(file_get_contents($file_name));
$part->setType($mime_type);

$mail = Solar::factory('Solar_Mail_Message');
$mail->addPart($part);
?>