As said, you can definitely convert most formats to .mobi or .azw3 for the Kindle using Calibre, and if you want to go to the trouble you can dress up the formatting and add a hyperlinked table of contents with MS Word and add a cover and metadata with Calibre to make it look like a real ebook. I actually did this once with a collection of files that I had in various text and .doc formats and turned it into a proper ebook indistinguishable from any other. I may have used some other free tools, too, can’t remember.
However, the question is whether it’s worth the trouble in the general case. I have both a Kindle Paperwhite and a small 7" tablet, and I find that I generally use the tablet for all document types except true ebooks. The Kindle really is optimized to be an ebook reader and is very very good at that, not much good at anything else.
It will read PDF’s but, as also mentioned before, not very well. PDF’s render much better on the tablet’s Adobe app. Another unfortunate problem with PDF’s is that despite a bit of hunting around and trying every conversion utility I could find, I’ve found no tool or combination of tools that can convert them to an ebook format for the Kindle at all well – they all result in terrible formatting problems (not the Kindle’s fault – the problem is with the tools).
I’m very very happy with the Kindle as an ebook reader but I don’t use it for anything else. And BTW, yes, you can upload files to it via USB, and in fact you can’t do it via wireless. Wireless only uploads books that have been purchased from Amazon. I don’t believe the Kindle supports any kind of user-accessible network file service.