Checks if the PDF contains unknown objects.
Unknown objects in this context are objects that are not defined in the PDF 1.3 standard. The PDF/X standards require all objects in the document to be “known”, or defined in the PDF 1.3 specification.
This avoids differences in output between older and newer RIPs, for example. Older RIPs may not be able to process certain objects, and produce blank pages, whereas a more recent RIP can deal with those objects correctly and will produce the intended output.