If you know HTML or have a good web authoring program, you can publish your book as an attractive web page or series of web pages. You can also put your book online as a plain text file, although that's less attractive and eliminates your ability to use graphics, hyperlinks, etc. Or, you can use an online form to collect payment information before linking customers to your online book or sending them a password via autoresponder. Allows for more attractive graphic design. Can include images, hyperlinks, audio, video, etc.
Printouts are of unpredictable quality.
With PDF, you essentially create a digital photo of your final book design. Readers across various computer platforms see the text, layout and graphics just as you created them, as long as they have a copy of Acrobat Reader with which to view your book. Just upload the PDF file to your website, and give readers a link or simply a file name they can access to download your book. Requires use of a separate software program such as Acrobat or PageMaker to design your book. Can be a large file requiring long download time.