Can't find httpd.conf

Ubuntu doesn't use httpd.conf as standard, instead global configuration stuff for apache is found in /etc/apache2/apache2.conf. You can create a httpd.conf in the apache2 directory, and load any further configuration from it by including the following line in /etc/apache2/apache2.conf.

Include /etc/apache2/httpd.conf

You don't need that file to configure apache, but you can create it if other software relies on it being there.

httpd.conf will be in /etc/apache2/.

/etc/apache2$ ls
apache2.conf  envvars     mods-available  ports.conf       sites-enabled
conf.d        httpd.conf  mods-enabled    sites-available
:/etc/apache2$ more httpd.conf
:/etc/apache2$

I need to change it

No, you do not. The documentation states:

httpd.conf: historically the main Apache2 configuration file, named after the httpd daemon. Now the file does not exist. In older versions of Ubuntu the file might be present, but empty, as all configuration options have been moved to the below referenced directories.

because when I try to open a .php page I see a download box

How would changing httpd.conf fix that? If you need config settings changed I would assume you need to alter apache2.conf or the virtual host in /etc/apache2/sites-enabled/.