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excaliber - the XMP code is put there by the built in
calibre Embed Metadata tool - you probably need to add the tool to a toolbar or assign it a shortcut (I have it as ctrl+e). I don't know if it gets injected into the PDF when it's exported - probably.
When you edit metadata in calibre it is written to the database but it is NOT written to the library format files - PDF, EPUB etc. A significant number of calibre users prefer to leave the library format files in their original state.
It is written into exported format files that are sent, saved or mailed. In the past the 'normal' way to get the current metadata into a library format file was to do a like-to-like conversion; but the Embed Metadata (introduced a year or two ago) tool is faster and safer, particularly for PDF's.
This ==>>
PDF-XChange Viewer, AFAIK it doesn't run native on Linux, it might under Wine.
BR