Only certain files can be transferred to your Kindle documents folder for you to read as an ebook. In the description of files for the newest Kindle available (the $79 Kindle) the first few files listed use the word natively. This means they are fine just the way they are. These native files are AZW, TXT, PDF, unprotected MOBI and PRC files. You would see these as a file extension at the end of the file name (like .txt, .pdf, .mobi, .prc etc).
HTML, DOC, JPEG, GIF, PNG, BMP through conversion means just that. You need something to convert these files to one of the Kindle native files, like .mobi or .prc to get a decent reading actual Kindle book file.
Two of my favorites are the Calibre software (free) and the Mobipocket Reader (also free). These should be on everyone's computer if they are serious about reading and want to truly utilize the wonderful versatility of their Kindle. Converting an html file or Word document can be as easy as dragging the file to the main screen of the software. Calibre or the Mobipocket Reader does the rest for you. Then it is just a matter of a few more tweaks in the software itself to add any finishing touches you may want (like cover art or editing the book title). I'll write more on these two wonderful tools individually soon, so stay tuned, dear readers.
You can read more about (and download) Calibre for your Mac or Windows computer at:
You can read more about (and download) Mobipocket Reader (Windows computer only) at:
You can also do a quick Google search for either of these and find links to them.