Do you get similar error if you edit the metadata from the file properties "details" tab (or from the Windows Explorer file details pane), instead of from the InfoEdit one? A different shell extension, the property handler, will be used in this case.
If the folder where you have the PDFs is being indexed by the Windows Search service try to rebuild the Windows Search index. If problem persists, and just for testing proposes, remove that folder from the list of folders to index. If the folder is included in a library, remove it from the library too and test again.