Amazon’s proprietary format for most of its Kindle devices is the classic AZW file. This is the file type associated with standard Kindle purchases made through Amazon.com. Most Kindle users see the filename extension, because Kindle book downloads are sent wirelessly to the user’s Kindle device, and are then directly accessible through the device itself. Amazon’s newer device, the Kindle Fire, uses the KF8 file type in a similar manner.
Kindle devices can open DOC and DOCX files by converting them to the relevant proprietary format via the standard file conversion method. Uses need only email those files -- as an attachment – to their Kindle email address. The conversion will happen wirelessly, and the files will show up in the Kindle home screen in the same manner as an e-book purchased through Amazon.com.
Kindle devices can open MOBI and PRC files purchased through a Mobipocket retailer as long as those files have no Digital Rights Management protection associated with them. Kindle users can add such files directly into their Kindle via a regular USB connection. Kindle users can enjoy these files without having to worry about putting them through any extra file conversion process.
Files with .pdf or .txt extensions can be read by Kindle devices without requiring any file conversion. Text files will be automatically converted to a Kindle-readable HTML format. It’s important to note, however, that you won't be able to changes the font sizes on a PDF without some further file conversion. Also, some PDF publishers have reported issues such as irregularly bolded text and inconsistent font sizes. Amazon’s website suggests Kindle users convert their PDFs to DOC files, and then use standard email conversion.