There are three main products I will mention that understand DOCX.
The obvious one is MS Word. It produces the definitive rendering of all DOCX files. Nothing is ever going to be exactly the same. By definition it is always correct. However it is not really designed for automated conversion and getting it to do this kind of thing is fraught with difficulty. On a legal level the EULA may confict with your chosen solution.
OpenOffice.org is a great product. The EULA is much more accomodating. The freeness is attractive. However, while it will produce a pretty good output for most DOCX documents it does not for all. While it is similar to MS Word it is not the same and this is something you may notice, particularly for more complex documents. Probably more importantly, again it's not designed for automated conversions and trying to get it to do this can be fraught and tiresome.
WordGlue .NET (on which I work) is a native .NET library that understands DOCX. It is designed specifically to produce output which is the same as MS Word. While I'm not going to say it is perfect (it's a big task) it is superior to OpenOffice.org in that it does actually attempt this as a specfic design decision. However probably the biggest advantage is that it is designed for high perfomance multi-threaded server side conversion. It's native .NET and thus low impact in terms of security.
Products like ABCpdf (on which I work) will integrate with these three applicatons to allow conversion direct to PDF. Why bother going via PostScript if you want PDF? However if you really want to save as PostScript you can do that too.
Or indeed you can write your own code to integrate with these products. Just be aware of the caveats above regarding fraughtness and tiresomeness relating to MS Office and OpenOffice.org. To get these things working unattended requires an awful lot of attention.