There are a number of flaws with this. The main one is that getElementById can by its very nature only give you a single DOM element. Therefore a for loop is pointless, because it will be looping over a single element.
The second flaw is that your two loops are identical, therefore you should combine them into one.
Finally, you are doing a comparison against a variable called
checked, which has not been defined anywhere.
If #pm was the single checkbox, you could do this:
var chk = document.getElementById('chk').checked;
document.getElementById('multiple_action').disabled = !chk;
document.getElementById('drop_button').className = chk ? 'drop_button disabled' : 'drop_button';