First, ensure that you are using Sign & Edit.

Then, select Overlay Text and click on the document to add custom text.

If Fill Out Form is not available, use Overlay Text tool to enter text.

When the Overlay Text Editor appears, enter text into the space provided. Change the font type, size, color, and style as needed. Select Save to place the new text on the document. 

:: Adjust placement of the text by clicking and dragging from anywhere within the Text Box.

:: Revise added text boxes by clicking within the box. 

Please note that all text created through Text Boxes will become temporarily invisible once (and if) you switch over to the Fill Out Form mode.