During my year-long job search, I often wondered if there was a particular file format that is best for submitting job applications. The short answer is: it depends. If a company specifies a particular format, adhere to their preference. Most places, however, give you a list of options. When I worked in human resources, I came to prefer PDF (Portable Document Format) for two reasons: first, a PDF file will by default open in your web browser; second, and most importantly, it preserves your formatting.* A Word document, on the other hand, may look quite different if it is viewed in a different version of Word than it was created in.
*NOTE: You may not have PDF creation software. Nonetheless, you can save Word documents as PDF files with Office 2010’s version of Word. For Office 2007’s version of Word, you can download from Microsoft an add-in to save Word documents as PDF files. For even older versions of Word, you can easily convert files (for free!) using a handy little website by the odd name of Zamzar.