Here are changes in pre-release 3.4 builds (https://www.sumatrapdfreader.org/prerelease):

• Command Palette with Ctrl - K
• configurable keyboard shortcuts https://www.sumatrapdfreader.org/docs/Customizing-keyboard-shortcuts
• better support for epub files using mupdf's epub engine. Adds text selection and search in ebook files
• -all-users cmd-line arg for installer for system-wide install
• added ability to search / translate selected text with web services
  we have few built-in and you can add your own via
  https://www.sumatrapdfreader.org/settings/settings3-4#SelectionHandlers
• added Annotations.TextIconColor and TextIconType advanced settings
• added Annotations.UnderlineColor advanced setting
• added Annotations.DefaultAuthor advanced setting
  Add advanced settings for default icon / color of Text annotation #2025
• Shift + i keyboard shortcuts inverts document colors
• u and Shift + u keyboard shortcuts adds underline annotation for currently selected text
  Underline Hotkey Shortcut to "u" #2157
• Delete / Backspace keyboard shortcuts delete an annotation under mouse cursor
• respect zoom level in PDF destination links 3aa18aa
• support .svg files
• faster scrolling with mouse wheel when cursor over scrollbar
• add -search cmd-line option and [Search("<file>", "<search-term>")] DDE command
• when showing document properties press Ctrl-d will show font information for PDF files
• support opening .heic image files (if Windows has heic codec installed)
• add experimental smooth scrolling (enabled with SmoothScrolling advanced setting)