Kindle devices support dictionary and
Wikipedia look-up functions when highlighting a word in an e-book. The
font type, size and margins can be customized. Kindles are charged by connecting to a computer's USB port or to an AC adapter. Users needing
accessibility due to impaired vision can use an audio adapter to listen to any e-book read aloud on supported Kindles, or those with difficulty in reading text may use the Amazon Ember Bold font for darker text and other fonts may too have bold font versions. It also includes other fonts for accessibility like
OpenDyslexic. The Kindle also contains experimental features such as a
web browser that uses
NetFront based on
WebKit. The browser can freely access the Kindle Store and Wikipedia on 3G models while the browser may be limited to 50 MB of data per month to websites other than Amazon and Wikipedia. Other possible experimental features, depending on the model are a
Text-to-Speech engine that can read the text from e-books and an
MP3 player that can be used to play music while reading. The Kindle's
operating system updates are designed to be received wirelessly and installed automatically during a period in sleep mode in which Wi-Fi is turned on. A user may install firmware updates manually by downloading the firmware for their device and copying the file to the device's root directory. The Kindle operating system uses the
Linux kernel with a
Java app for reading e-books.
Send to Kindle service Amazon initially offered a Personal Documents Service to add content to a user's Kindle which only worked via email. Documents were sent directly to the Kindle via WhisperSync. Later expansions added cloud library features and content management. The modern service is called Send to Kindle and is available through various means such as email, website, app, or browser extension. It allows the user to send files such as
EPUB,
PDF,
HTML pages,
Microsoft Word documents,
GIF,
PNG, and
BMP graphics directly to the user's Kindle library. When Amazon receives the file, it converts the file to
Kindle File Format and stores it in the user's online library (called "Your Content" by Amazon). Content added via Send to Kindle is added to the user library as Personal Documents by default, but some Send to Kindle interfaces allow users to send a document to a specific device and skip adding it to the library. The Send to Kindle service's personal documents can be accessed by all Kindle hardware devices as well as iOS and Android devices using the Kindle app. Until August 2022, in addition to the document types mentioned above, this service could be used to send unprotected and original version only
.mobi/.azw files to a user's Kindle library. Sending the file is free if downloaded using Wi-Fi, but, prior to 2021, cost $0.15 per MB when using Kindle's former 3G service.
Format support by device The first Kindle could read unprotected
Mobipocket files (MOBI, PRC), plain text files (TXT), Topaz format books (TPZ) and Amazon's AZW format. The Kindle 2 added native PDF capability with the version 2.3 firmware upgrade. The Kindle 1 could not read PDF files, but Amazon provides experimental conversion to the native AZW format, with the caveat that not all PDFs may format correctly. The Kindle 2 added the ability to play the
Audible Enhanced (AAX) format. The Kindle 2 can also display
HTML files. The fourth and later generation Kindles, Touch, Paperwhite (all generations), Voyage and Oasis (all generations) can display AZW, AZW3, TXT, PDF, unprotected MOBI, and PRC files natively. HTML, DOC, DOCX, JPEG, GIF, PNG, and BMP are usable through Amazon's conversion service. The Keyboard, Touch, Oasis 2 & 3, Kindle 8 & 9, and Paperwhite 4 can also play Audible Enhanced (AA, AAX). All Kindle models from the Kindle Paperwhite 2 and newer can display KFX files natively. KFX is Amazon's successor to the AZW3 format. Kindles cannot natively display
EPUB files. However, at least two methods allow viewing the content of EPUB formatted content on Kindles: • Specialized software like
Calibre allows EPUB or some other unsupported files to be converted to one of the supported file formats. • Kindles can be jailbroken to allow third-party software, such as KOReader which does support EPUB, to be installed. In late April 2022, Amazon announced that Send to Kindle would support EPUB, beginning in late 2022.
Multiple devices and organization An e-book may be downloaded from Amazon to several devices at the same time, as long as the devices are registered to the same Amazon account. A sharing limit typically ranges from one to six devices, depending on an undisclosed number of licenses set by the publisher. When a limit is reached, the user must remove the e-book from some device or unregister a device containing the e-book in order to add the e-book to another device. The original Kindle and Kindle 2 did not allow the user to organize books into folders. The user could only select what type of content to display on the home screen and whether to organize by author, title, or download date. Kindle software version 2.5 allowed for the organization of books into "Collections" which behave like non-structured tags/labels: a collection cannot include other collections, and one book may be added to multiple collections. These collections are normally set and organized on the Kindle itself, one book at a time. Collections can also be sent from one Kindle device to another. There is no option to organize by series or series order, as the AZW format does not possess the necessary metadata fields.
X-Ray X-Ray is a reference tool that is incorporated in Kindle Touch and later devices, the Fire tablets, the Kindle app for mobile platforms and
Fire TV. X-Ray lets users explore in greater depth the contents of a book, by accessing preloaded files with relevant information, such as the most common characters, locations, themes, or ideas.
Annotations Users can bookmark, highlight, and search through content. Pages can be bookmarked for reference, and notes can be added to relevant content. While a book is open on the display, menu options allow users to search for
synonyms and definitions from the built-in dictionary. The device also remembers the last page read for each book. Pages can be saved as a "clipping", or a text file containing the text of the currently displayed page. All clippings are appended to a single file, which can be downloaded over a USB cable. Due to the
TXT format of the clippings file, all formatting (such as bold, italics, bigger fonts for headlines, etc.) is stripped from the original text.
Textbook rentals On July 18, 2011, Amazon began a program that allows college students to rent Kindle textbooks from three different publishers for a fixed period of time.
Collection of users' reading data Kindle devices may report information about their users' reading data that includes the last page read, how long each e-book was opened, annotations, bookmarks, notes, highlights, or similar markings to Amazon. The Kindle stores this information on all Amazon e-books but it is unclear if this data is stored for non-Amazon e-books. There is a lack of e-reader data privacy — Amazon knows the user's identity, what the user is reading, whether the user has finished the book, what page the user is on, how long the user has spent on each page, and which passages the user may have highlighted. ==Kindle ecosystem==