Actually, the "other app" wouldn't have to produce PDFs at all, since those are just a midpoint to printing actual barcode labels.  For example, if we import CSV data into an Open Office spreadsheet, then it's just a question of page formatting and printing.  This is the approach we used at INFOhio (using a custom SirsiDynix Unicorn report and an Excel spreadsheet with some VB I wrote).  It was not particularly elegant, but it was good enough to print a couple million barcodes. 

Another approach would be to integrate with some widely available 3rd party print shop, like Kinko's.  I.E., figuring out what API was available for specifying (or pre-specifying) label stock and layout, and then just pushing CSV data.  We wouldn't necessarily need a PDF to just mail-order the barcodes printed commercially.  (A similar approach might be expanded to cover printing of patron ID cards.)