Hi!
The best way would be to define a constant in a separate include file which is put into every folder:
file includer.php
define("SITE_ROOT","../");
Put this file into every folder and include it on every page:
include("includer.php");
Now, include all your files like this:
include(SITE_ROOT."inc/class.pdflib.php");
This is very maintainable and you can put all your includes into a separate file and just include this file without worrying about possible errors.
Best,
Stas