Indeed, this is what was happening. I failed to test it from an outside source when I first noticed the problem. Locally, IE was passinging network-share information, allowing it to to download the pdfs. From an outside PC, even IE got the same error as other browsers,
Solution:
rsync script that copies the files to the webserver once a week. Does what we need, doesn't really hurt disk space too much, and gives us a good backup of all the pdf scans.
Thanks for everyone's input though. I'm still curious about sharing content lined from a mounted sambe share. It all seems to be in the permissions really, but being a windows networking dunce, I'll just have to keep digging.