Bridge comes with the Creative Suite and most point products. I believe it has been packaged with Acrobat since version 9, but I might be wrong.
It is a powerful file browser and organiser that allows you to easily edit metadata. Check out this video: http://tv.adobe.com/watch/learn-bridge-cs4/working-with-metadata-and-k eywords/
It is focused on image metadata, but the concept still applies to PDFs.
Adobe Bridge is a Creative Suite item and so is not included with the standalone license of Acrobat. You cannot buy Bridge separately, but the list of CS6 software that ships with Bridge is available here.
Bridge is a multi-format browser (as opposed to Lightroom, which only handles bitmap images and video). It will handle PDFs and other CS file types as well as video and audio.
Phil Harvey's ExifTool can read and write XMP metadata in PDFs. It works at the command line, which can be intimidating, but can it is very powerful. It can batch a series of operations to run on a set of files. (I should say I don't use ExifTool to insert metadata, but rather to see what I inserted via a custom panel in InDesign; the custom metadata is hard to check in Acrobat/Reader, so I use ExifTool to dump the info into a form easier for others to see.)
Message was edited by: David W. Goodrich
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