“Sioyek is a PDF viewer designed for reading research papers and technical books”
It uses the MuPDF engine, just like SumatraPDF does.
There is a recent discussion where the dev participates at Show HN: Sioyek – PDF viewer for reading research papers and technical documents | Hacker News
I like it a lot so far.
I wish SumatraPDF would implement many of its features.
I also wish SumatraPDF added easy ways for others to build features on top of it without being a C++ developer.
This will sound harsh but my view is that SumatraPDF has from a UX and feature perspective progressed quite slowly and in some ways even regressed over the last year. Specifically the new annotation system is even after more than one year of development in several ways worse to use and lacks features that the old annotation system permitted. Some will no doubt disagree with my assessment. But perhaps we can agree that it is good with open source competitors in this space.