You can copy documents in supported formate to your Kindle following these easy steps:
Connect your Kindle to your computer via USB cable. After drivers are installed, you can see your Kindle device as a disk drive
Navigate to documents folder on your Kindle device
Copy your files in supported format there (PDF/TXT/MOBI/PRC/AZW)
Safely eject the device
Your documents should now be visible along with your other eBooks in the home screen.