Change default value [GFX][thumbnails_png] = 1 for render pdf as png on frontend per default.
Now the default value is [GFX][thumbnails_png] = 0. So thumbnails are renderd to gif, having only 256 colors. Makes this still sense in 2016?
For PDF thumbnails this setting is also used, resulting in a gif having only 256 colors. This setting is used for the FE too, if someone is using a pdf in an image field, resulting in a 256 colors gif showing up.
Change default value to 1. ([GFX][thumbnails_png] = 1)
Or keep this as its now, and add new config value for pdf2img conversion, what about:
[GFX][pdfimgformat] = 'jpg'
[GFX][pdfimgformat] = 'png'
[GFX][pdfimgformat] = 'gif'