Find and replace text
Finding on the page
Open the find bar over the page with ⌘F, or from the Find menu, and type a word to see a live count of how many times it appears. Next and previous step through the matches and wrap around at the ends — ⌘G and ⇧⌘G do the same from a keyboard — and each match is tinted on the page so you can see where you are. This is finding within the open document; to find which document in your library mentions something, use Search your library instead.
Replacing text
On a document you can edit, a second row of the find bar adds Replace and Replace All (also on the menu as Edit ▸ Find ▸ Replace…, ⌥⌘F). The change is made in the page's real text objects, keeping each match's font, size, and colour, so the result stays vector and searchable rather than a flattened patch. A read-only document — one that's locked or signed — still gets find, just not replace.
Limits
Two limits come with editing real text. A match split across two separate text runs in the page isn't substituted, and replacing with a character that an embedded subset font doesn't include renders blank, because the glyph isn't in the file. Regular expressions, whole-word and case-sensitive toggles, and replacing text inside form fields aren't covered yet.