Settings/Preferences | Editor | General | Use soft wraps in editor — this setting affects all files.
You can disable or enable this option on per file basis (until it’s closed) via View | Active Editor | Use Soft Wraps
More recent version (compared to the versions available when this question was answered) also have these options available in the Editor’s gutter menu (right click in the area where the line numbers/breakpoints/etc are).
P.S.
Settings screen has very nice feature — quick search box (top left corner) — it does wonders. It allows you to quickly narrow the search scope of the specific setting by it’s name (keyword).
You can also use “Search anywhere” (Shift two times) as well as “Find Action” (Help | Find Action... Ctrl + Shift + A) to search for individual settings.
