Alfresco really doesn't have that much of an "OpenOffice API". The primary / only thing it has is the ability to use OpenOffice / LibreOffice for conversion of documents. You cannot use the ReST API to "just" transform documents. These documents must first be stored in Alfresco, and then you can use it to retrieve renditions of those documents, one/some of which may be PDF documents based on rendition definitions configured in Alfresco (by default there will be a PDF rendition for preview in Alfresco, if the document type allows for transformation to PDF).
With regards to adding data to template documents, Alfresco contains the necessary libraries to perform this if you were to implement custom actions, but no ready-to-use feature you can call via the ReST API.
