Amazon has released a Kindle firmware update that significantly extends the battery life of the electronic-book reader and adds the ability to read documents in the popular PDF format.

The PDF support is an important feature given the popularity of the format among businesses. Amazon's major rivals, Sony and Barnes & Noble, offer PDF support in their e-readers, the Reader and Nook, respectively.

Before the firmware upgrade released Tuesday, Kindle owners had to convert PDF documents downloaded from a PC into the e-reader's proprietary format. Native support for the format should reduce the hassle in reading such documents, which still have to downloaded to the Kindle through its USB connection.

Amazon has had a PDF reader in the larger Kindle DX, which is more geared toward business users than the basic Kindle with a six-inch display. However, neither device supports ePub, an open e-book standard by the International Digital Publishing Forum.

Not having ePub support means the Kindle and Kindle DX cannot read the half-million free, non-copyrighted library books available through Google Books. Many analysts believe ePub support is an advantage for the competitors from Sony and Barnes & Noble.

Besides adding a PDF reader, the Kindle firmware update also extends the battery life of the device to a week from four days with the wireless capability turned on. With wireless turned off, the Kindle can go up to two weeks without a charge.

The wireless capability is used in automatically delivering firmware updates and for buying and downloading e-books from Amazon. Barnes & Noble's Nook and some models of Sony Readers have the same capability for accessing the vendors' online stores.

Amazon released its latest update as competition heats up in the e-reader market. While Amazon is the current leader, Barnes & Noble appears to be doing well with the Nook, which debuted last month. According to Barnes & Noble's Web site, the device is sold out, and people pre-ordering a Nook today will have to wait until after the holidays. Barnes & Noble expects to resume shipping the device Jan. 4.

E-readers have proven popular among business travelers and avid readers. Forrester Research predicts e-reader sales this year will reach 3 million units, with 30% of the purchases occurring during the holiday season. E-reader sales next year could exceed 6 million units, according to Forrester.