Are you sure your using Moon+, since Moon+ does not support PDF.
You can write a script and use PDFTK to update the metadata.
Personally I think Alkdio and Mantano are lacking PDF readers. For PDF I typically use ezPDF or RepliGo since they support most if not all PDF standards. Also they don't read the metadata--that can be viewed as a bad thing but in your case it's a good thing.
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