Can you publish a digital version of your book on multiple distributor sites? Yes, provided you have not agreed to go with Kindle Select, or any other digital publisher which guarantees they are the only distributor for a 90-day period.
In fact, Mark Coker, creator of Smashwords, encourages it. “Get your book out to as many markets as possible,” he says. “You are the agent for marketing your book.”
Most sites require a membership (they have to know where to send your money), but membership is free. In most cases, your digital version must be in Microsoft Word format, i.e., *.doc, *.docx, or in a PDF. Some places require more. Here are a few sites to get you started.