If you use an e-reader, you probably want to copy over some of your PDF files so you can read them on the go. Most e-book readers, including the Amazon Kindle and the Barnes & Noble Nook, can interpret and display the contents of a PDF file: you simply drag the file over when your reader is connected to your computer.
PDF is a great format for most purposes, mainly because a PDF is designed to be completely portable and has a fixed layout no matter how you view it. That means everything stays in sync and in proportion. On an e-reader, that’s not always desirable.