The instructions for doing so (here) seemed straightforward enough, but I ran into a few problems. So here’s how I got PDFLib working with my PHP install on Fedora Core 4.

First we need to build PDFlib Lite from Source. Download it from here. Unpack it, then:
./configure
make
make install
Easy enough. Now:
yum install automake
yum install php-devel
pecl install pdflib
(Note: php-devel is required because we need phpize which is used by the pecl command.)

The PECL install will ask you a question: “Path to PDFlib installation?” This is where I ran into problems. You must put /usr/local/ and NOT /usr/local/include because the script is hardcoded to look inside the include directory automatically. Once PECL finds pdflib.h it will continue and finish compiling our shared object, pdf.so. The script will install it for us in /usr/lib/php/modules/
Now we just have to tell PHP to load the SO. Open up php.ini and add the following line:
extension=pdf.so
Save & exit, and restart your Apache with apachectl graceful. Now browse to a file containing the phpinfo() function and check to see if PDFlib is now active. If you see it, you’re good to go!

NOTE: I ran into a problem with RPM GPG Keys at the automake install step. I was getting the following error:
warning: rpmts_HdrFromFdno: Header V3 DSA signature: NOKEY, key ID db42a60e
public key not available for autoconf-2.59-5.noarch.rpm
Retrieving GPG key from file:///etc/pki/rpm-gpg/RPM-GPG-KEY-fedora
The GPG key at file:///etc/pki/rpm-gpg/RPM-GPG-KEY-fedora (0x4F2A6FD2)
is already installed but is not the correct key for this package.
Check that this is the correct key for the "Fedora Core 4 - i386 - Base" repository.
To get around this we need to import all the keys by run the following command:
rpm --import /etc/pki/rpm-gpg/RPM-GPG-KEY*
I found this solution in this thread.