The Toggle Ruler feature in the View tab is a “ruler” you can show and drag freely over any part of your document, which is frequently used to go through line items quickly and easily, especially when you need to read tables.
To show Toggle Ruler, click View > Toggle Ruler. You can click and drag Toggle Ruler almost anywhere on your screen. To hide/close Toggle Ruler, click Toggle Ruler in the View tab again, or click the Close icon 
on the ruler, or press the ESC key (when the focus is on the ruler).
To resize Toggle Ruler, put the cursor over the left or right edge of the ruler (top or bottom edge when the ruler is vertical). When the cursor changes into a double-arrow 
handle, drag the handle to resize the ruler.
Clicking the 
icon drops down a menu with more options, which allows you to do more settings:
- Horizontal/Vertical: specify the orientation of Toggle Ruler. You can also click the
icon on the ruler to change the orientation. - Reset position: Resets the ruler to the center of the current Foxit PDF Editor window.
- Color: change the color of the ruler.
- Opacity: change the opacity of the ruler.
- Close: exit/hide the ruler.