Would it be possible to implement a "choose PDF viewer" from our local files? I think being able to take whatever PDF viewer is native to Joplin now, and swap it out for PDF viewers some people already use for certain reasons. NightPDF would be a good reason to have the PDF reader swapable, you can change the settings to enable dark mode; Okular would be good for annotating PDFs. For example, I can manipulate the PDF in Joplin already, when viewing in Markdown. I can select text, copy and paste. With the toolbar of Okular, or NightPDF I'd be able to use what they've built within Joplin - like a small window within the text page.

Hopefully this is a good idea