To start adding text, click the ADD TEXT button (3).

Drag out a box the size of the area of text which you want to add to the PDF.

When you release the mouse, a textbox will be added to the PDF (this is sometimes hard to see on a white background PDF)

You can then start typing and your text will appear in the text box object that you just added.   This mode is called "edit mode" for the text box.

When you are done, you can click outside the text box area or choose the pointer tool.   This will deselect the text object and show you how it looks.   If you want to make changed to the text box, simply select the text box with your cursor, and then double click it.   This will put the text box back into "edit mode", allowing you to edit any text objects you added, including deleting text, adding new text, or editing existing text.

If you want to change the font and size of a text box or group of text boxes, simply select the text box(es).   Then use the top Selected Font tool bar (1).

Any changes you make on the tool bar will be reflected in the selected text boxes.   All newly added textboxes get their formatting and style from this toolbar as well.   So if you set it to font size 12, all newly added text boxes will reflect this and be size 12 font.

Clicking the APPLY TO PAGE (2) button will apply this font and style to all textboxes on the page.