Is it really that obvious? I genuinely don't know which services allow a permanent download that will continue to work in perpetuity and those that don't. My un

Is it really that obvious? I genuinely don't know which services allow a permanent download that will continue to work in perpetuity and those that don't. My understanding of file formats gives me some insight but - I shouldn't need to know that and some smart, technical people don't have that knowledge.