The Typewriter Annotation button allows you to type directly onto a PDF document, therefore allowing you to make notes as you read it. You can also change the colour and size of your font.
The Pushpin Annotation tool is very similar to the Typewriter tool, but instead of leaving the text in the document, it leaves a pin mark. You then click on the pin mark to see the text.
Comments allows you to comment directly in a PDF document. You just highlight some text and click on the icon that appears.
The box that then appears has a toolbar in it you can use and if the document is shared with others, you can collaborate and discuss with others the document through the comment box.
