The original Kindle supported only unprotected Mobipocket books (MOBI, PRC), plain text files (TXT), topaz format books (.tpz), and Amazon's proprietary DRM-restricted format (AZW). Version 2.3 firmware upgrade for Kindle 2 (U.S. and International) added native PDF support.[18] Earlier versions did not fully support Portable Document Format (PDF), but Amazon provided "experimental" conversion to the native AZW format,[33] with the caveat that not all PDFs may format correctly.[34] It does not support the EPUB ebook standard. Amazon offers an email-based service that will convert JPEG, GIF, PNG, and BMP graphics to AZW.[35] Amazon will also convert HTML pages and Microsoft Word (DOC) documents through the same email-based mechanism, which will send a Kindle-formatted file to the device directly for $0.15 per MB or to a personal e-mail account for free. These services can be accessed by sending emails to <kindleusername>@kindle.com and to <kindleusername>@free.kindle.com for Whispernet-delivered and free email-delivered file conversion, respectively. The file that the user wants to be converted needs to be attached to these emails. Users could also convert PDF and other files to the first-generation Kindle's supported formats using third-party software. The original Kindle supported audio in the form of MP3s and Audible audiobooks (versions 2, 3 and 4), which had to be transferred to the Kindle via USB or on an SD card.
Initially, Kindle 1 only supported the ISO 8859-1 (Latin 1) character set for its content; Unicode characters and non-Western characters were not supported. A firmware update in February 2009 added support for additional character sets, including ISO 8859-16.
Kindle 2 added support for Audible Enhanced (AAX) format, but dropped support for Audible versions 2 and 3. Using the experimental web browser, it was possible to download books directly on the Kindle (in MOBI, PRC and TXT formats only). Hyperlinks in a Mobipocket file could be used to download e-books[36] but could not be used to reference books stored in the Kindle's memory. Kindle DX added native support for PDF files.