Apparently only Firefox saves the source URL. In PDFs saved from Safari or Chrome on macOS I can't find the URL anywhere. Maybe I'm missing something.
/tmpᐅ pdftotext Add\ Comment\ -\ Hacker\ News.pdf - |grep http
https://news.ycombinator.com/reply?id=23232588&goto=threads%...
Safari 13.1 here, MacOS 10.14.6...