One of the best conversion programs available, as a free download, is the Calibre Program
Calibre will handle many different ebook formats and once you have added a book to your list you can then use the conversion tool to change it to your desired format. There are a couple of choices for reading books on a Kindle, you can either opt to save the file after conversion as a .azw file, which is Amazon’s own format or you can save it as a .mobi file (recommended) which is a generic format that can be read on a Kindle and for all intents and purposes is pretty much identical to the azw format. The only difference is that they use different DRM (digital rights management) schemes.
But don’t be confused by that if a book has had DRM applied you will not be able to convert it easily using this method, the whole point of DRM is that it prevents the simple sharing of ebooks. There are methods of removing DRM which you can work through if you want to, but I am not sure how good the results will be.
If on the other hand the files are not DRM protected then Calibre is a very good option for the file conversion process. Once the file has been converted to a format that Kindle can handle, all you have to do is copy the file to the ‘Directory’ on Kindle where the book files are normally stored and then it will be available in your book reading list.
The basics of using Calibre are covered in the video you will find at the bottom of this eBook Management post. The video describes the conversion of a file to ePub, but the principle and method is exactly the same, you are just selecting a different output format that Kindle can read.
When you connect your Kindle to a USB port on your PC a dialogue window should open asking you what you want to do with the device. Please make sure that you follow the manufacturers recommendations for safe connection and disconnection of a mass storage device when you do this, so that nothing gets damaged. You can opt to view files and folders and then navigate to the ‘Directory’ folder. This is normally where you place the file you have converted to a format that the Kindle can read as discussed above. You simply copy the format of choice to this folder to make it available on your book reading list as previously mentioned.