Imagick PHP image set question

[JohnSmith]

I read pdf file and go get set of images ($img_set) with 6 images in it.
Each property i supply is going towards particular image in set not entire set.

$img_set = new Imagick();
$img_set = readImage("test.pdf");
$img_set->setImageUnits(1);
echo "<br> First: ".$img_set->getImageUnits(); //1
$img_set->setIteratorIndex(3);
echo "<br> Third: ".$img_set->getImageUnits(); //0

Outputs:
First: 1
Third: 0

So if i want to resize vector graphics how do i do it? i know only option is to change dpi so i do

$img_set = new Imagick();
$img_set->setResolution(200,200);
$img_set = readImage("test.pdf");
echo "<br> ".$img_set->getImageWidth(); //1200
$img_set->setIteratorIndex(3);
echo "<br> ".$img_set->getImageUnits(); //600

Outputs:
First: 1200
Third: 600

How do i set higher resolution for any other image in set?