If the pdf file contains only images:
convert -density 150 -trim input.pdf -quality 100 -sharpen 0xx1.0
output.jpg
But, if the pdf file contains both images and text:
convert -density 288 model-of-the-standing-desk.pdf -background white -flatten -resize 80% model-of-the-standing-desk.png
You may also need to add this line:
<policy domain="coder" rights="read | write" pattern="PDF" />
inside the file /etc/ImageMagick-7/policy.xml
just before the line:
</policymap>
too.