Essentially, eBooks are produced in two different ways: Adobe® Portable Document Format (PDF), and Hypertext Markup Language (HTML), both with advantages and disadvantages.
Adobe® Portable Document Format (PDF)
PDF has the advantage of being a universal format that will maintain the specified layout and formatting, including images and links, independent of what application was used to create it. PDF files are very compact in size and can be easily distributed in print, as e-mail attachment, as download or burnt on CD-ROM or DVD. With the free Acrobat reader anyone can open the document regardless of their operating system, and can be printed with any printing device.
Hypertext Markup Language (HTML)
HTML eBbooks are in fact a group of web pages compiled into one .exe file for distribution. Unlike ebooks formatted in PDF, the design of an HTML ebook can contain live links, graphics, animated pictures, forms, JavaScript, embedded video, and more. They can contain pages that are navigated via hyperlinks, just like a web site. A distinct disadvantage is that users must trust the eBook as also viruses usually come as .exe files, and virus scanners block .exe downloads as being suspicious.