Is there a central document store in Drupal 6 or a module that does?
Sounds like the FileField Sources module is exactly what you're looking for.


  FileField Sources is an extension to the FileField module. The
  FileField module lets you upload files from your computer through a
  CCK field. The FileField Sources module expands on this ability by
  allowing you to select new or existing files through additional means,
  including:
  
  
  Re-use files by an auto-complete textfield Attach server-side files
  uploaded via FTP Transfer files from a remote server (cURL required)
  Paste a file directly from the clipboard (Drupal 7 only) Select
  existing files through the IMCE file browser