Enable apache file and directory indexing under Linux or UNIX

If a URL which maps to a directory is requested, and there is no DirectoryIndex (e.g., index.html) in that directory, then mod_autoindex will return a formatted listing of the directory.

Option # 1: Using Apache config file

Add directory option to Apache configuration file /etc/httpd/httpd.conf or /etc/apache2/apache2.conf:
# vi /etc/apache2/apache2.conf
Add following code:

Options Indexes FollowSymLinks

Save and close the file. Restart Apache:
# /etc/init.d/httpd restart
OR
# /etc/init.d/apache2 restart

Option #2: Using .htaccess Files with Apache

You can place config line Options Indexes in .htaccess file. Make sure .htaccess file support is enabled.
Change directory to pdfs
$ cd pdfs
Open .htaccess file
$ vi .htaccess
Append following apache directive:
Options Indexes
Save and close the file.