Transparent image watermark is causing error on pdf upload

Hi I have this plugin installed and working well. However when uploading a pdf to add to a post I am getting the following error:
Warning: imagesx() expects parameter 1 to be resource, boolean given in /home/~/public_html/wp-content/plugins/transparent-image-watermark-plugin/transparent-watermark.php on line 190

I am expecting that this plugin will watermark only image files and not other media, is this correct? And do you know why I am getting the above error?

thanks for your help.