You may use your computer operating system to move and create copies of the files on all your computers and mobile devices, as needed, as indicated by the License Agreement you agreed to when purchasing the eBook. Indeed, I highly recommend downloading the PDF file to the hard drive of your main system, and then distributing it to your other devices from there, as necessary. See this page.
All I ask is that you follow the Copyright law, both in practice and spirit: copies you make for your own purposes and convenience are okay, but you cannot distribute copies to others, put copies on Torrent networks, or sell copies to others.
