Ficbot,
What sort of documents are they? If they are Word docs or spreadsheets or similar, there should be no problem in physically transferring them to your Kindle, but you won't be able to do anything with them once they're there. You'll need to convert them to Kindle format first.
Apart from that, there's certainly no disadvantage to simply dragging your files onto the Kindle, for example from Windows Explorer. I do it all the time, without necessarily using Calibre, especially if they're files that I don't plan to keep in the long term.
If I've misunderstood your question, my apologies.
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