[pdf] ImageMagick security policy 'PDF' blocking conversion

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.