Highlight, underline, strike out — drag across words and the mark follows the text, not a rectangle drawn near it.
Pencil and pen — draw freehand; the pen smooths the line.
Rectangle, ellipse, line — drag a shape. Colour, thickness, fill, opacity and dashes can all be changed afterwards in the Edit palette.
Text Box — drag a box and type straight onto the page. The Text palette comes forward the moment you draw one, so the formatting is at hand: any font installed on your Mac (including its Bold, Light, Oblique and other variations — the font travels inside the file, so the page reads the same everywhere), size, colour, and even an outline around the letters. Click an existing box with the Text Box tool to reopen it, or double-click it with the arrow to edit the words in place — a real cursor, selection, and typing over a selection all work.
Notes — drop a sticky note; click it to open the bubble, edit it, change its colour. Right-click any spot on a page to Create Note there. Edit ▸ Show Notes on Page opens every bubble on the page at once. A note you close without writing anything is simply not added.
Sound — Edit ▸ Record Sound… attaches an audio clip to the page: record it with the microphone of your choice (Record / Stop / Pause / Play, with a popup to pick which input) or choose an existing audio file.
Stamps — Approved, Draft, Confidential, or a Custom… stamp with your own text and colour (Edit ▸ Add Stamp, or the Fill & Sign row).
Signatures — see §6.
Eraser — click or drag over any mark to remove it. Before you click, the mark that would go lights up along its own shape — a scribble along its stroke, a line along the line — so you always know exactly what you are about to erase.
Pick a mark with the arrow and it shows its box with handles: drag to move it, pull a corner to resize. Right-click it for Open Note, Note Colour, Delete. A click on a Swatches colour recolours the selected mark.
The Annotations palette lists every annotation in the document grouped by page, like a comments panel: click a row to jump to it, delete it, or edit a note's text.
Everything you add is one step: one undo takes it back, and the page underneath is untouched.