writing metatada on pdf

[giancarlo]

With this code I should be able to write a new keyword into document's 
metadata

PDDocument document = PDDocument.load("/root/saper.pdf");
PDDocumentInformation metadata = document.getDocumentInformation();
metadata.setKeywords("javakeyword");
document.setDocumentInformation(metadata);
But nothing has been written to the file! Suggestions?