Read Only Checkbox is clickable on Mac computer

Hello everyone!, I have a PDF with some text fields, checkboxes and dropdown lists, and created a button which once clicked, it makes all the fields "readonly=true". This works perfectly for all the fields in a Windows computer, but when I open the exact same document on a Mac computer the checkboxes and dropdown lists are editable (meaning the "readonly=true" is not working on the Mac),

Is there a workaround for this?, or is the code incorrect? (code below)

for(var i=0;i<this.numFields;i++)
{
    var cNm = this.getNthFieldName(i);
    this.getField(cNm).readonly = true;
}

Waiting your response and thanks in advance.

Regards,

Mario.