Sioyek is a PDF viewer with a focus on technical books and research
papers. It uses the mupdf rendering library.

Searchable
  Quickly search your previously opened documents, table of contents,
  bookmarks, and highlights. Sioyek can even generate the table of
  contents if the document doesn't have one!

Smart Jump
  You can preview or jump to references (e.g. bibliography, figures,
  equations, etc.) even when the document doesn't have links. You can
  also search references in google scholar with one click!

Portals
  You can create portals from parts of documents to other parts which
  are displayed in a second window. So when you are reading a paragraph
  that is talking about a figure, table, algorithm, theorem, etc. you
  don't have to keep jumping between the two locations.

Marks and Bookmarks
  Mark locations with one-character symbols and then quickly jump back
  to them. You can also create bookmarks which are like marks, but use a
  descriptive text instead of a symbol and can be searched.

Highlights
  Sioyek can highlight text with different colors. They are searchable!

Command Palette
  Discover sioyek commands and shortcuts using the searchable command
  palette.

Keyboard Focused
  While sioyek fully supports many mouse actions, it is still possible
  to use sioyek only using a keyboard. This includes functionality such
  as text selection too!

Synctex
  Sioyek autoreloads the document so you can preview the changes as you
  edit the latex file. You can also jump between the corresponding
  locations in PDF and latex files with one click.

Ruler
  Use a visual ruler to highlight the line currently being read. It
  reduces eye strain and combined with smart jump feature can create
  previews and portals without needing to click on the reference!

Configurable
  Sioyek is highly configurable. You can change all the keybinds and
  adjust the behavior of the application.

Extensible
  You can extend sioyek using external commands and scripts. For example
  you can add OCR, translation, and text to speech to sioyek.
