WordWriter does not support converting between file formats. This includes converting between DOC/DOCX and PDF. We generally recommend that you use a 3rd party component to convert Word documents to PDF files.
Note: Most Word to PDF converters require Office to be installed on the server in order to render the Word components before generating the PDF file. Microsoft does not recommend this:
Microsoft does not currently recommend, and does not support, Automation of Microsoft Office applications from any unattended, non-interactive client application or component (including ASP, ASP.NET, DCOM, and NT Services), because Office may exhibit unstable behavior and/or deadlock when Office is run in this environment.
For more details, see Considerations for server-side Automation of Office.
Word Services for SharePoint
If you are using SharePoint Server 2010, then one possibility is to use Word Services to perform the format conversion. This entails writing code to convert a Word document that is in a list or a document library in SharePoint.
This is a blog post that describes Converting Word Documents to PDF using SharePoint Server 2010 and Word Services. In this case, Word Services automatically converts a document to PDF when the Word document is added to a list.
Rainbow PDF by Atenna House has a server-based converter that does not require Office on the server. However, since Office is not installed, this component does not have the ability to render all Word document elements. Word documents rendered to PDF with Rainbow PDF experience loss of fidelity.
EasyPDF by BCL Technologies has a a PDF Creator SDK as well as server-side solutions that can convert Word documents to PDF. EasyPDF does require Office to be installed on the server, however it carefully manages the instances of Word to avoid hitting the pitfalls that normally happen when automating Office on the server.