Sending a document based on custom field input

I have a webform where a person inputs a batch number into a custom field. I need the follow up to send an email with a unique pdf file that corresponds with the batch number they enter. The possible batch numbers and associated pdf files will be a growing database. For now I could set up a decision diamond to trigger the appropriate sequence with corresponding pdf as an email attachment, but this isn’t really scalable as there will eventually be 100s of potential batches numbers.

Does anyone have any ideas as to integrations that would work for this? Perhaps an http post could trigger to a third party system that can send a download link based on the number in the custom field? I’ve used PlusThis, Parsey and Zapier pretty extensively, but a solution for this scenario isn’t coming to mind at the moment. Any suggestions would be appreciated.