ImageMagick Convert PDF to low resolution JPG file
You need two things that CodeIgniter probably doesn't support, so you have to use ImageMagick directly.
First, you have to set the resolution of the PDF for a high-quality result. On the ImageMagick command line, this can be done with the -density option. With PHP imagick, use setResolution.
To get rid of the black background, you have to flatten the PDF on a white background first. On the command line, use the options -background white -flatten. With PHP imagick, setImageBackgroundColor and flattenImages should work.
You can set quality and transparency of output picture in prefrences of 'image_lib' library. Please read http://ellislab.com/codeigniter/user-guide/libraries/image_lib.html and look for 'quality', and 'wm_x_transp' options.
I've run into a similar problem, which I solved for myself by calling GhostScript to create a png file (the jpg created wasn't high enough quality):
"gswin64c -r150 -dNOPAUSE -dBATCH -sDEVICE#pngalpha -sOutputFile=" + strTitle + "-%%02d.png " + strTitle + ".pdf"Then converting the jpgs to pngs (using ImageMagick):
mogrify -format jpg *.png