listing checkmarked items

Confused: So if the unpack already figured out the checkboxes, what is the "isset" do here?

The isset() is not 100% necessary; it's just error checking to make sure that the PDF record still exists. Since if you assign a PDF to an "assign" record, then delete the PDF record, the "assign" record will still contain the deleted num, but it won't exist in the $pdfsByNum array.