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 need an external tool to create a XSD file from your XML file.
After that, just load the XSD file in your process via the Schema Settings of your xfaForm variable.
That brings up the nodes in the Path Builder.
Should I up to date the XSD? I mean, should I add field name to XSD file while I add a field to the PDF?
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.