I looked at the issue in code and here is what I can tell:
The embedded object in this OOXML document is not a ready to use PDF document. It is in fact an OLE object that represents a PDF document. In other words, it is a PDF document wrapped into some OLE object wrapper.
When you request Aspose.Words to extract an OLE object, it just does what you ask for - it extracts the OLE object. This includes the outer OLE wrapper with your PDF document inside. The output file as a result is not immmediately a valid PDF document and hence you might not be able to open it in some applications that expect pure PDF and don't like the OLE wrapper.
The design idea in Aspose.Words was that it is impossible to know how to automatically unwrap all types of OLE wrappers and extract the pure embedded document. The idea was for the user then to use the extracted OLE object on their own. So as far as the code is concerned there is no problem in Aspose.Words.
But from user requests we now see that we need to add unwrapping to our functionality. In one of earlier versions we added automatic unwrapping of Outlook email messages. This time it looks we are going to add automatic unwrapping of PDF documents. We will also look at other possible embedded document types to unwrap.
Here is a note for future requests: If you have a similar issue, just report to us and attach your document - we will add automatic unwrapping to Aspose.Words so what you extract will be the pure document without an OLE wrapper data.
Will release PDF unwrapping later today.