Answer
To change the Scale Ratio of a measurement in Foxit PDF Editor and Foxit PDF Reader for objects that have not yet been measured, please follow the steps below:
1: Open the PDF file in Foxit PDF Editor/Foxit PDF Reader.
2: Cick on the "Measure" button under "Comment" tab, and select one of the measurement tools you want to use, such as Distance or Perimeter.
3: The Format panel should appear on the right side of the screen. If the right format panel doesn't open, please refer to this article to learn how to bring it up.
4: In the Format panel, you will find the "Scale Ratio" option. Adjust the precision by changing the values and measurement units in the Scale Ratio fields.
5: Start by clicking on a point where you want to begin the measurement, then complete the measurement by double-clicking at the point where you want to end the measurement.
6: The measurement result will be displayed on the measurement markup that you have just drawn.
For existing measurement markups, if you want to reapply a new Scale Ratio for them, you can use the calibrate tool in the format panel. Here's how:
1: Go to the File tab in Foxit PDF Editor/Foxit PDF Reader>"Preferences">"Measuring", check the option “Use Scale and Units From Document (When present)”, and click on "OK" to save the setting.
2: Follow steps 2-4 from the instructions above.
4: In the Format panel, click on the "calibrate" tool to select it. Use your mouse to select two points on the PDF page, which will open a new dialog box called "Calibration". Enter the new page scale in this dialog box, and check the option "Store Scale in page" or "Apply to all Pages", and click on "OK".
Note: If only the "Store Scale in page" option is selected, the newly calibrated page scale will only be applied to all measurement markups on the current page. If the "Apply to all Pages" option is also selected, the newly calibrated page scale will be applied to all measurement markups on all pages of the current PDF file.