Deciding it is not worth finishing

Stopping is a skill and nobody teaches it. If after the first pass the paper is not answering your question, has been superseded by later work, or is written so badly that you are reconstructing rather than reading, put it down. There are always more papers than there is time.

Keep a one-line note on everything you abandon and why, so you do not rediscover and re-abandon the same paper in three months. That file becomes surprisingly valuable, and it is most of the difference between reading widely and reading in circles.