OpenLoops gives a nice tip for this - just send the document to your own gmail address, but instead of sending to
yourname@gmail.comyou now send to
yourname+docs@gmail.com... the string "+docs" is ignored by gmail - and you can use a simple filter rule to add a label to all those mails...
Update (March 11th 2008): Some people asked, wether it is safe to store documents in GMail. IMHO it is not. I would currently not store any sensitive information there (nor at any other online-location!). But not every document is really sensitive...