The format of an ebook determines the nature of the digital package in which it is distributed. During the short history of the ebook, various new formats have been developed and others may evolve in the future. But, as with any maturing technology, a point will probably be reached where one or two formats dominate, to the exclusion of all others. Fortunately, it is becoming increasingly easy to re-package existing content in alternative formats, so there is now less risk associated with making an early commitment to a particular format, even if it ultimately fails the test of time.

The important considerations when choosing an ebook format are:

• Compatibility with the platform used by the target audience
  
• Ease of installation and use by your readers
  
• The ability to support any special features required by the intended content

All common ebook formats now support flexible text formatting and page layout, embedded graphics, hyperlinks and convenient navigation.

The two formats most commonly used by independent ebook authors are PDF and HTML (also known as EXE format). They are favored for their ease of creation and widespread acceptance by the ebook community. Although other formats exist (MS Reader, Adobe ebook and Palm Doc etc), they typically entail more complex compilation procedures and include features more applicable to mass publishing.

The PDF format, developed by Adobe Systems Inc, can be viewed on Windows and Unix systems, as well Macs and even handheld computers. An ebook in PDF format is viewed using the free, downloadable Adobe Reader software.

Although a specific version of the Adobe Reader application is required by each target platform, a single common PDF file format is applicable to all users, regardless of their platform. This cross-platform compatibility is probably the single most important factor in the huge popularity of PDF as a publishing format. Over 500 million computer users are currently equipped to read PDF documents.

A PDF ebook can contain thousands of pages and the inbuilt hierarchical navigation system (known as bookmarks) makes it as easy to find a particular chapter or page as in a printed book. For this and other reasons, the PDF format is generally the professional's choice.

HTML ebooks are created by combining a set of HTML pages (i.e. web pages) into a single file. The resultant ebook can only be viewed on Windows computers, but since the majority of buyers fall in this group, most marketers do not consider this a major limitation. Unlike PDF (and other formats), HTML ebooks do not require any additional pre-installed software to read them. Most variants are dependent on Internet Explorer, but since this is installed automatically with Windows, the issue is of no consequence to most users.

A common feature of many HTML ebooks is the ability to customize certain pre-defined aspects of their content, even after creation and distribution. This is known as re-branding. It is very popular among online marketers, particular those producing promotional ebooks, because it enables the ebook medium to be used as a viral affiliate marketing tool.

On the downside, the need for multiple content files means that HTML ebooks are more cumbersome to create, making the format less well-suited to large authoring projects. Also, since HTML ebooks are directly executable by Windows, they are a potential source of computer viruses. Although there have been few incidents of virus problems originating from the ebook industry, the ever-present threat means that some prospective readers are wary of downloading ebooks in this format.