My understanding is that this is largely because you're fighting an adversarial format provider in Adobe, I've read a few papers and journal entries on file format polyglotting, with some focus on PDF and approaches are constantly shifting in nature due to Adobe mooting pathways to success, I think it's partly for security and also IMO partly for obscurity as PDF is a horrific format in all reality except for human visual interpretation. Many organisations and industries ONLY produce public data via PDF and it makes the parsing of that information a far more difficult task (again, I suspect by design). After trying a few parsing options for PDF, I've come to hate it as a format. Luckily though, some options from a few cloud providers seem to be really hammering the problems complexity down, but the cost of the solutions can be very steep.