The best way to know is to try and open the file. If you cannot view it, your device will usually give you an error message: “you need a PDF/ePub reader to access this file.” Another indicator that your device cannot open a file is when a file you know is PDF or ePub appears without an image after you download it. Your device may ask you if you want to open it using a different program, but you won’t have any luck in most cases unless another program can read the same kind of file. If a file is compatible with your device, it will usually show the icon of that program – the most straightforward example is probably the blue “W” on the front of a Microsoft Word document.