I am currently writing a PHP-based web application that requires a search function for all documents, some of which are PDFs. I have found various third-party libraries which can convert most of them correctly; however, for PDFs of the 1.7 specification, it simply outputs whitespace. I have confirmed this issue on multiple PDFs and across many different third-party PDF-to-text converters. I even downloaded the trial versions of some paid PDF converters, and even these only outputted whitespace.
I have read answers on here previously which link to either tutorials or libraries or command-line executables which all claim to convert PDFs to text - and, indeed, do so for the 1.5 specification, but none of them work with 1.7.
I am at my wit's end. Is there something I can do to convert a 1.7 PDF to a 1.5 PDF? Is there a library I'm missing? At this point, even just a command-line executable would be acceptable, although of course I would prefer to be able to see the code myself.