If you only expect to read your ebook on a computer (not on a phone or special ebook device), then you should choose a PDF-based ebook. This is because PDFs display the pages of a book exactly as they were designed by the book publisher. In other words, a PDF will usually maintain the design integrity of the original printed book. With other formats, pages can look different – sometimes radically different – when displayed.
PDFs sometimes display awkwardly on small screens, while some ebook formats are designed especially for reading on smaller screens. If you have a smart phone, PDA or special ebook reader device, then you should choose a format suitable for that device.