Open files downloaded from PHP

I'm trying to open files with this function. For files *.pdf and *.txt works fine, but if the file is *.doc or *.odt when downloading and opening it, strange characters appear.

$filepath = '/var/download/prueba.doc';
    if(file_exists($filepath)) {
            header('Content-Description: File Transfer');
            header('Content-Type: application/octet-stream');
            header('Content-Disposition: attachment; filename="'.basename($filepath).'"');
            header('Expires: 0');
            header('Cache-Control: must-revalidate');
            header('Pragma: public');
            header('Content-Length:'.filesize($filepath));
            flush(); // Flush system output buffer
            readfile($filepath);
            exit();
        }

What am I doing wrong? Is there any way to open any type of file once downloaded ?. I do not need to open them online, I need to be able to download them and once downloaded, I can read them on my PC.