How to use phpthumb with codeigniter

How to use phpthumb with codeigniter

While I can find other questions like this, I can find none that actually
has the answer. I want to be able to do something like this
<img src="phpthumb_path/phpthumb.php?src=test.pdf&amp;w=500">
The goal of which is to take a pdf that I know the loctaion of and output
it to the screen as a thumbnail. My application is written in codeigniter
and I can't figure out how to get phpthumb working. I am aware that there
a lot of other ways to do this, but unless they are codeigniter specific
with complete instruction as to how to do them they aren't of any use to
me. I've seen the code above on several forums, with no explanation as to
how to integrate the library. I would think having the php thumb path
would be enough, but I can't seem to get it to work and I have verified
that I have the correct path.