Downloading is saving something that's online for later offline use. I think that's pretty easy to tell apart from reading something online?
If this is something you can currently distinguish, I’d be very interested in hearing more!
(b) Why does something being illegal imply that you have to be able to distinguish it from something legal in every instance? They could just only apply it when they can make a case that it was done in an illegal manner. It's not like we don't already have laws where intention makes a strong difference, despite the fact that you can't always establish intentions.
For example, if a web browser has a semi-permanent cache by default for all pdf files, and the browser downloads a pdf to this cache, it's not possible to determine if the user "downloaded" it (ie. deliberately, to access later or repeatedly) or "streamed" it (ie. accidentally accessed it and closed it after viewing the content)
The fundamental issue is that politicians and the general public do not understand how software works. Many applications access and store data by default which the user has not implicitly asked them to do. If the law is determined so broadly, it can put any user with an application that accesses content without somehow validating that it is legal (how you do that without accessing it I do not know) into a grey area which the state can choose to exploit/prosecute with if needed.
Again it confers power to those that understand how systems work. I'm not complaining, it benefits me but we continue to drift toward a class-based system where the technical class gains ever increasing benefits over the rest of the plebs. We're sleepwalking into an increasingly stratified society.