If you want to provide shortened URL's to files you can achieve it using the RewriteRule in your sites .htaccess file.
Shortened file path: http://www.mysite.com/images/somefilename.jpg
Actual path to file: http://www.mysite.com/sites/default/files/somefilename.jpg
Shortened file path: http://www.mysite.com/pdf/somefilename.pdf
Actual path to file: http://www.mysite.com/sites/default/files/pdf/somefilename.pdf
The RewriteRule solution
Ensure this is placed in your Drupal sites .htaccess file within the mod_rewrite rules area, ie. inside <IfModule mod_rewrite.c>
RewriteRule ^images/(.*)$ /sites/default/files/$1 [R=301,L] RewriteRule ^pdf/(.*)$ /sites/default/files/pdf/$1 [R=301,L]
PS. make sure that your shortened URL path doesn't match any of the root folder names that appear within your core Drupal code base, eg. /sites, /themes, /profile...