Instead of a "specific container" make that a "specific related record" that happens to have the container field storing the PDF.
Then selecting a value from the drop down can select the specific related record so that it's PDF (or other inserted file) is accessible.
What version of FileMaker are you using?
If FileMaker 12, do you want to have interactive access to the PDF file?
This is key to some details in how you set this up, but your basic relationship would be:
Personnel::__pkPersonnelID = Immunizations::_fkPersonnelID
and then one field in Immunizations provides the values shown in your value list while a container field in Immunizations stores the PDF file.
Yes I have Filemaker 12. I want staff to be able to delete their old immunizations and insert their new ones. I have found though, that if it is inserted as a PDF I can't delete it but if I put it in as a jpeg I can.
With the related records approach, they can delete the entire related immunization record.
Set field [Immunizations::containerfield ; "" ]
Can clear the field to delete the inserted file or file reference.