Linking to files inside Drupal
I'm not sure what kind of answer you are looking for ...

If you just want these files stored remotely, but accessible from drupal you may want to look at these modules:


Filedepot
MediaMover
Migrate


I actually have a similar criteria on a site. Serve pdf file listings from a non-drupal directory to allowed users -- essentially a simple directory listing. Here is how I did it in D7 with a bit of custom coding.


I make a symlink from a remote filesystem into like DRUPAL_SITE_DIR/sites/default/files/REMOTE_MOUNTPOINT
Make a custom module that uses something to scan for files and show them to the enduser. You can use D7 file_scan_directory if you like, I actually opted for Symfony's Finder Component. The advantage to Symfony's finder is things like "show files of X size", "sort by something" or doing more complex search filters (which I will need) or truely different types of remote files (FTP, S3, etc). Example code for a module poorly named "views_fs" can be found in this public Gist.


Hope this info helps.