Is there a good reason that this method doesn't return the bool success/fail from QImage::save()?
No good one - maybe it is because PDF is by far the most important format ( vector graphics ! ) and here Qt doesn't offer any return code ( the same for SVG and Postscript ).

Is there another way to obtain a status?
Well, QwtPlotRenderer::renderDocument is only a convenience API - you can simply copy the 10 lines dealing with QImage to application code.

Another option might be to compare the date of the output file. That works for all output formats, but of course doesn't detect all types of write errors.

Uwe