You could export the data from the PDF (as xml) and use a XML utility tool to generate a schema from the xml data file, but we don't provide tools that will create the schema for you.
That doesn't mean the xPath won't work. If you don't have a schema, you just need to know the structure since you won't see the nodes from the xPath Builder window. But the expression will still work.
You should keep your XSD file up to date so that your data bindings in the PDF are correct. You can also embed your XML Schema (your XSD file) into the PDF when you create the Dataconnection (in Form Designer).
Afterwards you don't need to manually load your schema settings (in the Workspace) because its embedded in the PDF.