One of the most common things you can do with a PDF (other than read it or print it, of course) is mark it up with notes, comments, and additional text blocks. To that end, PDFpen provides a variety of tools.
Persistent tools: The markup tools, and, in fact, all the tools other than the selection tools, are persistent by default. That is, they remain active once you’ve chosen them. For example, if you choose the Text tool (see below), every click on a page creates a new text box until you choose another tool. You can change this default behavior with PDFpen’s General preferences.