DjVu are about half the size of PDF docs.

http://www.djvuzone.org/

Could you allow for attachments of this type. Their browser plugins for IE and Netscape/Mozilla work great.

The any2djvu server will let anyboby that wants to convert .pdf, scans, or pics into .djvu to do so free.

http://any2djvu.djvuzone.org/

The DjVuZone says that LizardTech has given to non-profits but you wouldn't need to convert anything as that could be done by the server by the posting party. You don't even have to install anything.

 

We don't allow PDF attachments to begin with, only the following:

jpg gif png jpeg bmp

Is there a reason you are requesting this? Do you work for them?