Calibre is helpful if you buy books from other sources, not just Amazon. People with large libraries like it for its organizational tools. If you have books that need to be "format shifted" because they won't work on your Kindle because they didn't come from Amazon, Calibre can do that with the correct plug-ins.
If you only buy books from Amazon, you may not need Calibre. There are ways to get your document on your Kindle using email.
Explain in detail what you're trying to accomplish, what type of document you're trying to get to your Kindle, etc. Include which specific Kindle you have.
Here is Amazon's help page on sending documents to a Kindle:
https://www.amazon.com/gp/help/custo...69&sr=13-1-acs
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