Call this from Edit > Tools > Measure and choose Distance Tool from the drop-down list. A dialog box with three tabs appears; make choices in the Settings and Mark up panels, then draw a line over an object in the PDF whose distance you want to measure.
View tab
The Measurement shows:
Distance: As you are drawing the line, it dynamically shows its length so far.
Angle: Shows the current angle of the line you are drawing related to the X axis. No signs or direction of rotating are considered.
In the Cursor Location area, the X and Y coordinates of the current cursor position are dynamically displayed relative to a start point of X=0,Y=0. No signs are considered.
Settings tab
Perpendicular Measurement sets your line to be drawn orthogonally (at right angles to the X or Y axis).
Choose from Inside or Top caption styles for the length display.
Units Settings allow you to set scaling for your measurement. A map or scale drawing should indicate its scale, e.g. 0.7 inches = 1 mile.
In the Snap Settings area you can specify that your measuring should snap to points, edges, or arcs in your document. Adjust snap sensitivity and color.
Markup tab
Use Markup: Choose this setting if you want the current measuring to be stored with your PDF document, otherwise all results are lost as soon as you start drawing a new line. Type a text into the text field to appear in a pop-up comment window when you hover with your pointer over this measurement.
Measuring Color: Sets the color for your measurement lines.
In the Leader area you can set the length of lines indicating the beginning and end of the object to be measured. Click Set Length as Default to keep these settings. Offset determines how far away the measure line is from the cursor.