If your app requires the ImageMagick executables (for example, your app's configuration page asks for a path to ImageMagick) rather than the ImageMagick PHP extension, you can install the ImageMagick package by logging in to your server as root and running this command:
sudo apt-get install imagemagick
You will then be able to run commands, such as convert, from the command line.
In order to allow ImageMagick to process PDF files, you must SSH into your server as root and edit the following file:
Locate the line:
<policy domain="coder" rights="none" pattern="PDF" />
Comment out this line by replacing it with the following:
<!--<policy domain="coder" rights="none" pattern="PDF" />-->