FreePDF: Changing the Background color is possible in FreePDF only for newly inserted text boxes.
A text box can have a transparent, white or black background. The last two options are useful for hiding elements behind a text box.
In this example, we would like to cover the name Wilhelm Korte on the page with the new name Hans Keppler.
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▪If you place the text on the existing name, both names will be visible:
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▪Choose the ribbon command Format | group Format | Background color 
| White to give the text box a white background.
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For the sake of simplicity, FreePDF only offers transparent, white and black as background fills.
Tip: If you need a different color, simply use the drawing tools to insert a rectangle in the desired color and place it in the layer between the new and old text.