Impressive! Seems like you can't just use PDFBox out of the box (no pun intended) and need to write some custom code specific to the PDF itself per the chart-parser commits[1]
Author here; well, PDFBox is good for simple text stripping. If I wanted to print all the text on the PDF, that would be very straightforward and not much code. However, the PDF chart here is in essence a representation of structured data. I wanted to get the content in that format so that I could both serialize to JSON plus have an SDK to boot.