So my current project needs me to have PDF files loaded from the files directory located in the sites directory. I have all the proper permissions and for the files directory and some PDF files load while others do not. I'm confused why some work and why some do not and i need to get this working as this is the last small issue on this project.

PHP Code:
$file       = isset($file)      ?' Attachment: <a target="_blank" href='.$base_url '/sites/default/files/'.$file.'>'.$file.'</a>':' '