You can load your own Word documents onto it. Go and check Calibre which is a sort of free library manager available for Windows and Linux.(An ebook iTunes!)It allows you to convert various documents and books to mobidoc which is an ebook format which can be loaded onto the Kindle via the USB connection. Cut 'n' Paste Web page stuff into Word (or Open Office saved as doc files) to make your own reference documents.
You can also load PDF files but I find them a bit tiresome to view as you zoom in on them to scale up parts for better viewing and have to Pan about.
I'm not sure if it's been fixed but you may also have to edit a python driver file associated with Calibre to change a product code to 3 (I think) to allow Calibre to identify the latest Kindle. Check the Calibre website FAQs If you can pick Kindle 3 from the devices menu you don't need the edit. You should be able to check out Calibre first without buying a Kindle by the way!
You can search for words, bookmark and highlight passages within a document (Not sure of PDFs though)
Calibre is available from he Ubuntu repositories. The Kindle User Guide is available from Amazon as a PDF file, it also comes loaded on the Kindle.