Microsoft Word documents (.doc) are commonly used for résumés since they allow you greater control over layout and design. Some sites and email programs, however, may not accept Word formatting, so email the document to yourself or a friend to make sure the formatting is correct before you send it to an employer.
Plain text (.txt) files are most useful if you intend to email your résumé or submit it to an online résumé bank. These files, however, do not retain formatting such as italics or underlines.
The Adobe Acrobat Portable Document Format (.pdf) will retain the design of your résumé, but it cannot be edited after you download it.