The Imagemagick security policy seems to be not allowing me perform this conversion from pdf to png. Converting other extensions seem to be working, just not from pdf. I haven't changed any of the imagemagick settings since I installed it... I am using Arch Linux, if the OS matters.
user@machine $ convert -density 300 -depth 8 -quality 90 input.pdf output.png
convert: attempt to perform an operation not allowed by the security policy `PDF' @ error/constitute.c/IsCoderAuthorized/408.
convert: no images defined `output.png' @ error/convert.c/ConvertImageCommand/3288.
Well, I added
<policy domain="coder" rights="read | write" pattern="PDF" />
just before </policymap> in /etc/ImageMagick-7/policy.xml and that makes it work again, but not sure about the security implications of that.
As pointed out in some comments, you need to edit the policies of ImageMagick in /etc/ImageMagick-7/policy.xml. More particularly, in ArchLinux at the time of writing (05/01/2019) the following line is uncommented:
<policy domain="coder" rights="none" pattern="{PS,PS2,PS3,EPS,PDF,XPS}" />
Just wrap it between <!-- and --> to comment it, and pdf conversion should work again.
For me on my archlinux system the line was already uncommented. I had to replace "none" by "read | write " to make it work.
来源：https://stackoverflow.com/questions/52998331/imagemagick-security-policy-pdf-blocking-conversion