pngalpha device does not work for multi-page pdfs

[Helen]

Specific pdf and AI files render better when the sDevice is changed to pngalpha in delegates.xml. The problems is that this will only render the first page of a multi-page pdf file even when you specify the range using the [] notation after the input filename you only get one png file as the output. I am using version 6.5.9.3 of ImageMagick

[Drarakel]

This is a known behaviour of Ghostscript. See here:
http://www.imagemagick.org/Usage/formats/#ps_reading

The 'ps:color' delegate is used rather than 'ps:alpha' by default because the 'pngalpha' ghostscript device only supports a one page/one image and PDF's generally are multi-page.