Is it acceptable? Result quality showed "Good". The Z average is 127. Is material RI essential? Some papers showed that it might be dust and result acceptable.
Possibly. What is the material? What i the PDI as well as the z-average? Why not show the report? Conversion to volume (for which the material RI is needed) will remove the top 5000 nm peak (it's unlikely as material this size is almost certainly settling within seconds) . Are you using plastic cuvettes? Inspect these for signs of scratching - you may have flare causing the higher end peak. Repeat in glass cuvettes. For the intensity distribution only the continuous phase (dispersant liquid) RI is required.