I've always used my E Ink kindles as a place to keep personal documents for handy reference while away from my home office. For example, as a photographer I like to have camera manuals on my Kindle in case I forget some arcane setting. As a professional pilot I like to keep a copy of the FAA regulations and Airman's Information Manual on the Kindle for reference. The Kindle Fire is even more useful for personal documents because of its vivid color display. On the Fire I can store charts and other color graphics in PDF format and zoom in to see the details.
Off course, you can transfer files by connecting the Fire to your computer with a USB cable, or by emailing the files to your @kindle.com address. But an easier way is to take advantage of your Wi-Fi connection:
WiFi File Explorer is a free app that installs on the Fire and allows you to browse the Fire's folders and files from a Web browser on your computer, as long as the Fire is on and connected to your home network via Wi-Fi. I suggest you try the free version first, and if you like it, buy the pro version. WiFi File Explorer Pro adds more features.
If all you need is the ability to transfer files, have a look at Dropbox. After installing Dropbox on your computer and on the Kindle Fire and creating a Dropbox account, a Dropbox folder appears on the hard drive of your computer and on the Fire. Any files you place in the Dropbox folder are available on the Fire and any computer running Dropbox. The Dropbox app on the Fire lets you browse, open, share, and delete files in the Dropbox folder. To move files such as music files to another folder on the Fire, you'll need an app such as ES File Explorer, which is free.