If you [south:: Prefer to store useful idea elements], then you need to ask yourself: "Is what I'm reading a..."
If not, then discard it. Don't worry about adding it to your PKM.
If it is, well, you'll need to decide if this particular element of information goes into your PKM. That's up to you.
Although it may not be evident how a piece of information is useful at the outset, it's still important to decide what information should be retaiWriting depends on your ability to read Upon Your Ability To Read]]]. If you try and retain everything, you'll retain nothing. If everything is important, nothing is. Etc.