You need to explain what ‘single image’ you are hoping for. A 600 page PDF turned into a ‘single image’ to fit completely on a single screen doesn’t seem likely. Maybe you mean a typically-legible-width super-tall series of pages? Even with that- would you want that ‘single image’ to have page numbers, headings and other typical PDF details included? Would people curse you for the time it takes to load a monstrous single image onto the screen- or wishing t had decent control over how to search within it, or control how to print sections of it? You goals and expected purpose for such a ‘multi-page single image’ will be important in picking the method for creating what you seek. It’s quite doable, in many ways, but identifying your needs is the first step in picking a method.