This is a simple example of Multiple File Upload
To achieve similar through Magento's Form Library, we will modify its code a bit in this way
Modify getElementHtml method of lib/Varien/Data/Form/Element/Abstract.php
public function getElementHtml()
{
    if($this->getType()=='file' && $this->getMultiple())
        $_multiple = ' multiple';
    $html = '<input id="'.$this->getHtmlId().'" name="'.$this->getName()
         .'" value="'.$this->getEscapedValue().'" '.$this->serialize($this->getHtmlAttributes()).$_multiple.'/>'."\n";
    $html.= $this->getAfterElementHtml();
    return $html;
}
and then just pass new attribute multiple=>true in you field declaration like this 
$fieldset->addField('uploadpdf', 'file', array(
    'label' => Mage::helper('promotionsoffers')->__('Upload PDF'),
    'name'  => 'uploadpdf[]',
    'multiple'=>true
));