Thanks for your inquiry. Please note you cannot create 508 compliant PDF documents while converting Word document to PDF using Aspose.Words. Because it mimics MS Word behavior and MS Word does not support 508 compliant PDF document generation.
However, we have an API Aspose.Pdf to manipulate PDF documents. But I am afraid it does not support the requested feature yet. We have already logged a feature request(PDFNET-26512) to create and edit 508 compliant(accessible) PDF documents. We have linked your post to the issue id and will notify you as soon as it is implemented.