Note the passage that matters while you read the paper, and the source URL stays attached to it.

Finding, reading, finding again — we remove the last one
When a passage matters, note it in the panel beside the page instead of switching tabs.
When a page supports summaries, scan one first and read the passages you need yourself.
Make a category per project or subject and file your notes into it.
Every note carries the page URL, so citing it later doesn't mean retracing your steps.
Other ways people use the same tool
Note what a tech blog or newsletter actually concluded, with the original URL attached.
Write in the side panel while a lecture plays. The lecture URL stays with your note.
Write what comes to mind in a side panel next to what you're reading. The page URL is saved with every note.
Add it to Chrome and use it on the source you already have open.
Free to use