When a document like a sales invoice or purchase order is send by email, the subject of the email and the name of the PDF will always be "Document type - Document no."
It would be nice if (via new setup) some variables (like "Your reference" or "Ext. Document No." from the order/invoice) could be added to the name and subject.
Also, the subject and document name should be in the language of the customer/vendor, not the users working language.
That has also been mentioned in another Idea.