You can download an issue in the magazine only three times in a period of twelve hours since you have bought it. After 12 hours, the download link becomes invalid. If you open the download link three times (maybe on the purchase receipt page, then on the email we send you with the link), you will run out of the three downloads limit and will see that message.
The recommendation is to right click on the link for the issue and to Save the PDF as a file on your device, rather than opening it and reading it from the site. If you open the PDF in Acrobat Reader, then you have the option to save it locally, rather than having to re-download it.
Lastly, if you log in into your profile, you will be able to download again all the magazines you have purchased: once again, save the PDF to your local disk so that you can access it while offline and without limitations.