I’m thinking about it similar to how American law enforcement is required to store digital evidence, in particular the requirement that it’s “unmodified”. Per my understanding, that would disqualify, e.g., a .jpg that was converted to a .png from being submitted as evidence.
Obviously a journalist isn’t law enforcement but a good one will still gather and retain evidence. They also don’t necessarily need to be as strict as prosecutors but it’s a good standard to try to emulate. To that end, a PDF wouldn’t do.
I wouldn’t doubt that these assumptions are overly generous to the reality of their evidence-gathering; it’s a bit idealistic and the implementation is lacking, at best. But if that’s their point behind the screenshot instead of inline text, I still think they’re coming from the right place. They mostly just have a bad implementation.
But at the same time, I get where you’re coming from, even assuming more responsible intentions. A bit more applied critical thinking in such a context likely results in a better implementation. That is largely why the above assumptions seem so generous.