Full screen view in LibreOffice?

CTRL+Shift+J toggles Fullscreen mode, more info here: LibreOffice Help - Full Screen

On menu, go to View > Zoom > Zoom.... There, on View layout, choose Columns and put there how many pages side by side you want.

Then Ctrl+Shift+J to Fullscreen (or from the menu View > Fullscreen; Esc to exit).

Then Ctrl+Wheel mouse up/down to adjust the page size as you want (or before going to Fullscreen, set menu View > Zoom to adjust in "width and height" or as you want).