The popular proprietary file formats such as those employed by Microsoft Office, Adobe and Open Office all provide varying degrees of accessibility for up to date magnification and screen reader software.
Producing accessible documents
The production of accessible documents using these and other formats is of benefit not just from an accessibility viewpoint. The creation of well designed, properly structured documents is for the most part a more efficient use of the production tools. Not only that but well structured documents lend themselves to easy conversion to accessible formats such as clear print, large print, synthetic audio or braille often with little further editing.
Microsoft Office
Used appropriately, the Microsoft Office suite is capable of generating accessible information from Word documents and Excel spreadsheets through to simple and more complex multimedia presentations using Powerpoint for a wide variety of users and purposes.
PDF and ODF
Portable Document Format (PDF) and Open Document Format (ODF) are a means of saving documents so that they may be accessed on machines which do not necessarily have the software to read the original source files. For example, a Word document may be saved in PDF format and then accessed on any machine which supports the free Adobe PDF reader.
Increasingly files are being accessed on devices other than computers such as e-book readers and mobile phones requiring file formats to be transportable across different operating systems.
Which format should I use?
Your choice of format will depend to a great extent on what software you have available to you. Rather than think in terms of format it is perhaps more important to be aware of the accessibility and other benefits associated with creating well designed and properly structured source files.