The EPUB document format is admittedly better for its adjustable fonts and formatting, and it's recognized by Apple's iBooks app. But saving your own writing to EPUB format isn't exactly simple.still requires connecting via iTunes with a sync cable.
PDFs are the Web's most versatile format, used for everything from digital magazines to personal files. Most word-processing programs including Google Docs and operating systems have very simple ways to convert any file to PDF. The iPad reads PDFs nearly as well as it does EPUB files: graphics and text look good on the large iPad screen, although page-turning isn't as smooth and conveniences such as in-book note-taking aren't common yet. A number of good PDF readers exist on the iPad, includingand , but now that iBooks can import and read PDFs that's nearly become moot.
Even better, you can import PDFs into iBooks directly via an attached file in an e-mail, eliminating a need for syncing.
For my own writing (script drafts, articles, plays), PDFs are a great way to retain complex formatting without a lengthy education in EPUB conventions. I still read novels in EPUB or Kindle format, but PDF is how I take my own library on the go.
Maybe someday, importing EPUB files will be handled as effortlessly, too.