None of the RISC OS PDF viewers can display transparency or PDF colour graduations. I seriously doubt this is going to change in RiScript - both things would require them to write their own graphics engine, which is pretty unlikely bearing in mind that only ArtWorks and Vantage ever managed to display transparent vector graphics, both being big and expensive graphics packages. For technical reasons that are beyond what can be discussed on this forum, the graphics engine required to display PDF file exported from ArtWorks 2.5 would have to be much more complex than even the graphics engine used in ArtWorks 2.5 itself!
As I indicated in my recent e-mail on the ArtWorks mailing list I expect to have a better (though not perfect) solution soon. The above PDF file, for instance, is displayed reasonably well, though with visible colour banding and some artefacts that only go away when zooming in, but significantly better than what !PDF and RiScript can manage.