Why do you need a .Net tool? Just create a bat file and call it from code using OpenOffice http://www.tech-faq.com/convert-word-to-pdf.shtml

You can do this if the file is postscript using ghostscript which is GPL. You should be able to call Word using a system call to save as postscript and then convert to pdf with gs.

I use Word's COM API to print to a PostScript file then convert to a PDF with GhostScript as part of web service. The Office COM API is pretty horrible to call from C# (finally fixed in C# 4.0 with support for optional ref and support for missing arguments) but it works. However, there are some issues with this technique - for one, there's no way to hide dialogs. Most noticeable is the print dialog, but if Word crashes and then has to display the file recovery dialog the next time it starts up or the file you're opening is locked for editing for some reason your app will hang until the user clicks through Word's UI. In a carefully controlled environment you can get away with these things, but probably not if this is software the user interacts with normally.