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There are some tasks that are difficult using the Open XML SDK, such as repagination, conversion to other document formats such as PDF, or updating of the table of contents, fields, and other dynamic content in documents. Word Automation Services is a new feature of SharePoint 2010 that can help in these scenarios. It is a shared service that provides unattended, server-side conversion of documents into other formats, as well as some other essential pieces of functionality. It was designed from the outset to work on servers, and can process high volumes of documents in a reliable and predictable fashion.
I've co-authored a paper, Developing with SharePoint 2010 Word Automation Services, which is published on MSDN.