Unused attachments don’t get cleaned out immediately because it’s somewhat of an expensive operation to figure out which are unused. That’s because the same attachment could be referenced from different cells (and even in different attachment columns or different tables). They do get scanned and cleaned out periodically – about once an hour – so Raw Data should reflect the reduced size after a little while.
Can I ask which service you used to generate PDFs from Grist data? I feel this wish comes up fairly often, so I’m interested in recommendations!