From the river site.
Personal Document Service Via Whispernet
Kindle makes it easy to take your personal documents with you, eliminating the need to print. Each Kindle has a unique and customisable e-mail address. You can set your unique e-mail address on your Manage Your Kindle page. This allows you and your approved contacts to send Word, PDF documents, and pictures to your Kindle via Whispernet for a small fee—see details. Kindle supports wireless delivery of unprotected Microsoft Word (DOC, DOCX), PDF, HTML, TXT, RTF, JPEG, GIF, PNG, BMP, PRC and MOBI files. Some complex PDF and DOCX files might not format correctly on your Kindle.