I don't think you need to install pdflib-devel package.
Install
1- pdflib RPM
2- php-pecl-pdflib RPM
3- restart your httpd-e-smith service
I compiled php-pecl-pdflib RPM in my SME7.x-DEV environment so the answer to your question is YES, it should work for SME 7.3.
More info about pdflib-devel package
http://rpmfind.net//linux/RPM/dag/redhat/el4/i386/pdflib-devel-6.0.1-1.2.el4.rf.i386.html
PDFlib is a C library for generating PDF files. It offers a graphics
API with support for drawing, text, fonts, images, and hypertext. Call
PDFlib routines from within your client program and voila: dynamic PDF
files! For detailed instructions on PDFlib programming and the
associated API, see the PDFlib Programming Manual, included in PDF
format in the PDFlib distribution.
This package contains the files needed for compiling programs that will use
the PDFlib library.