Your Kindle Paperwhite can't read every file that's on your computer. A file has to be in a Kindle-compatible format so that the device can display it. There are options for converting files, but the following files should not need any conversion to be readable on your Paperwhite. The following list describes the Kindle-friendly file types:
AZW is the Kindle format, so most files you download from the Kindle Store will have an AZW extension. Note that some files might have a number, such as 1, 2, or 3, after the AZW.
MOBI, a file format developed by Mobipocket, is widely used and compatible with the Kindle Paperwhite. Note, however, that secure Mobipocket or secure MOBI files do not work on a Kindle Paperwhite.
PRC is equivalent to MOBI and is the standard file format for the Kindle family of e-readers for content that comes from sources other than Amazon.com.
TXT is a simple text file, such as one you might create with Notepad.
PDF: Your Kindle Paperwhite can display files in Adobe's Portable Document Format (PDF).
Files in these formats are generally readable natively — that is, without needing to be converted — by your Kindle Paperwhite. So all you have to do is transfer the files onto your Kindle Paperwhite, and get reading.
If you use Windows Explorer or Finder on a Mac, you may see files with the POBI extension on your Kindle Paperwhite. Curious about these? Do you have any periodicals (newspapers or magazines) on your device? If so, they'll have the POBI extension. POBI is a variation of MOBI and enables the display of articles and sections in periodical content.