Is it possible that the metadata you need is consistent across multiple forms?

I haven't used the PDF Metadata activity, but I had a similar issue with the Fill Out PDF Form activity - where I had 5 fields that I needed to populate and it could potentially be across dozens of different documents (it's actually over 100 different ones now), I didn't want to load all of the documents to Workflow, and upadte them every time I made a change to the original PDFs.  But I was able to make tweaks so that they all used the same 5 fields (and managed them slightly differently internally if needed).  Then I just loaded a simple version of my PDF that used those 5 fields.  From that point, I was able to make the Workflow work on any of those documents with the 5 fields, regardless of which document it actually was.  It doesn't check that it is the specific document, it just confirms those specific fields.

Maybe the metadata will be the same - if you are able to create a PDF that contains every type of metadata you might want to work with...