What does it show?

The pdf contains black font or graphics which is faulty in terms of printing.

How to interpret the result?
The pdf contain black font, graphics which is not 100% black, but made by blending of 4 colors.

PDF 100% black font or graphics fault

What does the warning mean?
Checking 4-color black font
These fonts or graphics could lack definite contour when printed. The readability of text deteriorates.
Not 100% black text.
The converter shown below corrects the fault if all the four colors of the font or graphics is in the range 50-100%.
If the font is included in the pdf as image, then the fault cannot be corrected.
How the problem could be resolved?
Converting 4-color cmyk font to 100 % black.
cmyk black font, 100% black font