PDF 100% black font or graphics error.

Checking 4-color black font
What does it show?
If we intend to merge two or more PDF document in a single PDF.
How to interpret the result?
The PDF contains a black letter or graphic that is not 100% black, but made of a mixture of 4 colors.
cmyk black font, 100% black font
Not 100% black text.
What does the warning mean?
When printing, these letters or graphics may not have nice contours. The readability of the text deteriorates.
How the problem could be resolved?
The following converter corrects the error if all four colors are between 50-100%.
If the letter or graphic is included as an image in the PDF, the error cannot be corrected.
Converting 4-color cmyk font to 100 % black.
How to use it?