Microsoft PDF add-in for word seems to be the best solution for now but you should take into consideration that it does not convert all word documents correctly to pdf and in some cases you will see huge difference between the word and the output pdf. Unfortunately I couldn't find any api that would convert all word documents correctly. The only solution I found to ensure the conversion was 100% correct was by converting the documents through a printer driver. The downside is that documents are queued and converted one by one, but you can be sure the resulted pdf is exactly like the word docuemtn. I personally preferred using UDC (Universal document converter) and installed Foxit Reader(free version) on server too then printed the documents by starting a "Process" and setting its Verb property to "print". You can also use FileSystemWatcher to set a signal when the conversion has completed.
