We'd like to support layers in the PDFTron viewer, but unfortunately this is not supported or planned by PDFTron in combination with the streaming XOD format. We chose for using XOD because:
1. it minimises load on the browser
2. the content is streamed, so the user does not need to wait for the entire file to download
PDFTron can also view the native PDF (this is currently not implemented in Meridian), but that would negate the advantages. In this case, layers will be fully supported.
With XOD, we could partly support layers by creating separate PDFs. It would then not be possible to freely select layers to view, only to select from predefined sets of layers.