ImageMagick is a free available collection of programs for image manipulations. You can essily play with 200 different extensions images with ImageMagick. You can install it with the help of simple commands on unbuntu.

sudo apt-get install php5-imagick
sudo apt-get install graphicsmagick-imagemagick-compat
sudo service apache2 reload

exec('convert "/var/www/html/preview-image/pdf/TEST2.pdf" -colorspace RGB -geometry 200 "/var/www/html/preview-image/pdfimage/document.jpg"');

In above line we are going to fetch a cover image from the pdf file. You can perform other different options as well like file format conversion,Color quantization etc

It is totally depends upon the command line. We have different types of options in command to achieve the goal. Take another simple example

convert rose.jpg -resize 50% rose.png

It is useful to resize the image. You can combine the different operations to match the requirement.