PDF files can contain a piece of arbitrary metadata, often in XML format. This is typically stored in an uncompressed stream, so that other applications can read it without having to decode the whole PDF. To set the metadata:
cpdf -set-metadata data.xml in.pdf -o out.pdf
To remove any metadata:
cpdf -remove-metadata in.pdf -o out.pdf
To print the current metadata to standard output:
cpdf -print-metadata in.pdf