An easy way to create PDF files is to use iTextSharp. It is an opensource PDF library that allows to you generate PDF files on the fly. It's great if you need to dynamically create a PDF using dynamic data. Also, I found that this is a great tool to easily make barcodes. You can create common barcodes such as Code 128, Code 39, EAN 128, and Code 39 ext.
You can download a copy of the library here. There are also a load of tutorials which can be located on the site.
I've included a copy of a small project that I've whacked together using the library which shows how to create a barcode in a PDF.
Davinder - 1/23/2010 Thank you for your post! I was looking for something like this.
merritt - 8/13/2010 thank you for the example project. it's a bit trickier when you want to apply to an existing pdf. there is a discussion on stack overflow about stamping images, which is applicable for barcodes: http://stackoverflow.com/questions/583629/how-can-i-insert-an-image-with-itextsharp-in-an-existing-pdf