// I am pretty sure this one should work, but couldn't find how to do it in php:
convert -density 288 in.pdf -resize 25% out.png

// thus this one:
convert -density 600 in.pdf -threshold 15% -type bilevel -compress fax out.pdf

// Also found this one:
convert -density 288 image.pdf -resize 25% -threshold 50% -type bilevel image.tiff

$img = new Imagick('path/to/document.pdf');
$img->quantizeImage(2,                        // Number of colors
                Imagick::COLORSPACE_GRAY, // Colorspace
                50,                        // Depth tree
                TRUE,                     // Dither
                FALSE);                   // Error correction
$img->writeImage('path/to/output.png')