One of the more practical uses that I have for my Kindle Fire is that I store all of the user manuals for the various hardware that I install and troubleshoot on the device. I had them all stored on my nettop, but it can be a hassle to lug around my netbook all of the time. The Fire fits nicely in the back pocket of my jeans, or the inside pocket of my jacket.
To add PDF files, or other documents to your Kindle Fire, all you need to do is find out what your @kindle.com email address is. You can easily find this out by going to the home screen, then clicking DOCS. At the top of the screen it will say "send documents to firstname.lastname@example.org (the xxxxxx being whatever username that is set up for your Kindle)
Now all you have to do is open up your email client on your PC, and send the PDF file(s) or other documents as an attachment and they will appear in the DOCS folder of your Fire. Your Fire will need to be connected to the internet in order to download these documents onto the device.