I do this all the time. I belong to a writers group and I always email the monthly critque group submissions to my Kindle email account, so that I don't have to print them out and add to my paper clutter. I've read 400-page first draft novels on my Kindle. It's certainly a lot handier than shlepping around the entire manuscript!
There are two ways to do this: You can email the Word document or PDF as attachments to your Kindle email account (yourname@kindle.com) for a small fee, around 99 cents. I don't mind paying for the convenience. You can also email the documents to your free Kindle email account, which will also convert them, but not always right away; that one is WiFi, not 3G, so it depends on whether or not you're in a WiFi area: yourname@free.kindle.com. There is one more option: Attach the Kindle to your computer as an external USB device, and transfer the documents yourself. That works, but they don't always look as good, because they're not converted to Kindle format.
Edited to add: Obviously, the free.kindle.com email really is free; you only get charged for docs sent to the kindle.com account.
I'd suggest asking someone who has a Kindle if they've ever done any of the above, and asking if you can take a look at the documents to see if their appearance is OK with you. I'd be happy to answer any specific questions.
