When downloads are delivered to the user, the files are read using the PHP readfile() function, but because some hosts do not allow files to be ready via URL, this will occasionally fail. The EDD_READ_FILE_MODE constant is available for these scenarios.
If a host does not allow readfile() to access files via URL, then define this in functions.php or any custom plugin:
When this constant is defined to “header”, Easy Digital Downloads will use header(“location:” . $file_url) for directing the user to download the file. This is not as secure so should only be used when readfile() does not work.