I've had to use a couple of libraries that don't have composer support. What I did was create a folder called 'libraries' in 'app/' directory in the root of you Laravel 4 project. Then I added a line of code to app/start/global.php:
<?php
ClassLoader::addDirectories(array(
app_path().'/commands',
app_path().'/controllers',
app_path().'/models',
app_path().'/database/seeds',
app_path().'/libraries', // ADDED this line
));
Note: I've not done this before with HTML2PDF, so can't be 100% sure it will work. I think the directory structure of files in the app/libraries folder will have to follow the namespacing used in the library (if they use namespaces that is)