D634233590 Debug ID for retrieving metadata for pdf.. link to pdf also https://www.crs.org/sites/default/files/unaccompanied-minors.pdf

It won't retrieve any data as opposed to retrieving the wrong data

this is the case with around half of the pdfs in my library any advice?

If this is a fairly typical document for your library, this isn't particularly unexpected, though. Essentially, the more academic the source, the better retrieve metadata will work.

Just to be clear -- you can obviously add metadata for those PDFs manually: right-click --> Create Parent Item and then select an appropriate item type & fill out the relevant fields.