Not sure if this existed in the beta version of iChat A/V, but I found that if you start an iChat discussion in Panther with a buddy, and then drag a PDF file to that window, a direct connect will start and the PDF will appear as something you've 'said.' You can then browse that PDF inside the iChat window. If it's a text documnet, you really can't 'read' it as it shrinks it too much, but you can scroll thru the whole thing. Even the scroll wheel works on it!
[robg adds: At least in the iChat in Panther, you can read it if you resize the iChat message window - the PDF (any image, in fact) will dynamically resize as you increase the size of the window.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20031117134358659