When using the built-in PDF viewer, the URL bar contains, instead of the URL of the PDF file, a URL referring to the extension. For example, when clicking at
one gets, in the URL bar:
chrome-extension://oemmndcbldboiebfnladdacbdfmadadm/https://www.w3.org/2019/Talks/Amersfoort-IH/Presentation.pdf
This may be a problem if one wants to, e.g., copy-paste the URL of the PDF file. The same extension is used in other browsers, and I have not seen this behaviour before…