There doesn't seem to exist any direct way how iWork files (.iwa) could be converted to anything else. These .iwa files are found inside compressed documents from iWorks (.pages, *numbers etc.) and replaced the previously used XMLs that were utilized for the document structure.
All this means that iwa to pdf with files for former iWorks documents is not possible. If you want to convert your document, you will need to convert it as whole, not just a part from inside.
According to some sources, it may be possible to somehow decode the IWA files (they feature Snappy compression) and rebuild some kind of "old" version of iWorks documents, but that seems like highly unreliably and complicated way.
There is also potentially another iwa to pdf conversion sought by users that try to convert old IBM Writing Assistant documents (.iwa) to PDF. Sadly, we do not know about any reliable software that can manage this conversion.
