dlfilePlugin.php

<?php
// should be at /craft/plugins/dlfile/dlfilePlugin.php
namespace Craft;

class dlfilePlugin extends BasePlugin
{
    public function getName()
    {
        return Craft::t('DL File');
    }

    public function getDescription()
    {
        return TemplateHelper::getRaw("Allows a file to be accessed at a given route");
    }

    public function getVersion()
    {
        return '0.1';
    }

    public function getDeveloper()
    {
        return 'Eric McNiece';
    }

    public function getDeveloperUrl()
    {
        return 'http://emc2innovation.com';
    }

    public function addTwigExtension()
    {
        require_once __DIR__."/extensions/dlfileTwigExtension.php";
        return new dlfileTwigExtension();
    }
}

dlfileTwigExtension.php

<?php
// should be at /craft/plugins/dlfile/extensions/dlfileTwigExtension.php
namespace Craft;

class dlfileTwigExtension extends \Twig_Extension
{
    protected $env;

    public function getName()
    {
        return 'dlfile';
    }

    public function getFilters()
    {
        return array(
            'dlfile'  => new \Twig_Filter_Method($this, 'dlfile', array('is_safe' => array('html')))
        );
    }
    public function getFunctions()
    {
        return array(
            'dlfile'  => new \Twig_Function_Method($this, 'dlfile', array('is_safe' => array('html')))
        );
    }

    public function dlfile($id=null)
    {

      // Access the url with ?debug=true to see a dump
      $debug = craft()->request->getParam('debug', false);

      try {
        $file = craft()->assets->getFileById($id);

        if ( ! $file) {
          return false;
        }
        $source = $file->getSource();

        $sourcePath = $source->settings['path'];
        $folderPath = $file->getFolder()->path;

        $assetFilePath = $sourcePath.$folderPath.$file->filename;
        $contents = file_get_contents($assetFilePath);

        if(!$debug){
          craft()->request->sendFile($assetFilePath, $contents, array(
            'forceDownload' => false,
            'mimeType' => 'application/pdf'
          ));
        } else{
          $output = $assetFilePath;
        }
      }
      catch(Exception $e) {
          $output = false;
      }

      return $output;
    }
}