Send the documents via email to your Kindle's free email address:
yourkindle@free.kindle.com. The kindle.com email address should be on the Manage Your Kidle devices page. If you have a 3G Kindle, you have to use the free.kindle.com address. If you have a wi-fi only Kindle, you can just use the regular kindle.com address, since they only charge if files are delivered via 3G. Even if they charge, it's only .15 / megabyte, which isn't very much. Most ebooks are far less than a megabyte in size.