Use the Word Automation Service to convert the files to pdf.
You can do this by creating a job to run every so often to check the document library for documents that have been updated and generate a pdf file for the coresponding docx file. Perhaps create an extra DateTime site column for the document library to store when it has last been checked by the job and compare that with the last updated date of the file.
This article can be very helpful
http://blogs.msdn.com/microsoft_office_word/archive/2009/12/16/Word-Automation-Services_3A00_-What-It-Does.aspx
Good Luck
David Opdendries
----------------------
blog