cf: https://stackoverflow.com/questions/52998331/imagemagick-security-policy-pdf-blocking-conversion
Due to ImageMagick security policy, just remove the lines from /etc/ImageMagick-6/policy.xml
:
<!-- disable ghostscript format types -->
<policy domain="coder" rights="none" pattern="PS" />
<policy domain="coder" rights="none" pattern="PS2" />
<policy domain="coder" rights="none" pattern="PS3" />
<policy domain="coder" rights="none" pattern="EPS" />
<policy domain="coder" rights="none" pattern="PDF" />
<policy domain="coder" rights="none" pattern="XPS" />