I would love to see thumbnails for PDFs. What's the best method to use currently? I read PDFs with Sumatra, but if I need to install another reader to see thumbs, that's OK.
Anything that works in File Explorer should also work in Opus.
I had heard Adobe got their act together and Adobe Reader did thumbnails again, but I tried it recently and that did not seem to be the case. It's possible none of the files I tested with actually had thumbnails in them, perhaps, as it wasn't the main reason I was installing Adobe Reader, just something I quickly tried at the same time.
PDF-XChange used to be a good bet for both a preview handler and thumbnails, although I haven't tried it recently.
I would also like to see PDF thumbnails. Is there integrated features in Dopus nowadays for this? If not, whats the best method in 2020? I'm using Nitro Pro PDF application.
PDF thumbnails depend on your PDF viewer (and possibly other PDF software that might fight with your PDF viewer for the relevant registry settings; usually the last installed/updated/repaired wins if there is such a conflict).
If they work in Explorer, they should work in Opus as well. If they don't work in Explorer, ask the people who make the PDF viewer for help fixing things (or to confirm if they include thumbnail support at all; some viewers may not).
I have thumbnails with SumatraPDF
I have them also with PDF-Exchange Viewer.