Use the Dimensioning tool

Use the Dimensioning tool from the Drawing Markups toolbar to add lines between objects or that span distances in PDF documents, such as technical drawings or blueprints. Type in the Value box to add a comment that appears above the line, or double-click the markup to open a pop-up window for your comments. Use the Hand tool to adjust or move the line, and use the Properties Bar to change the line thickness and other attributes.

You may also want to:

Select, move, edit, and delete comments