What does it show?
Is there any crop size and bleed surface of pdf.
How to interpret the result?
If the Trim value is not 0 x 0 mm, then crop size exists.
If the crop size of pdf is smaller than its full size (e.g. 169 x 231 mm => Trim: 148 x 210 mm), then pdf has bleed surface.
PDF bleed and crop size
What does the warning mean?
Pdf has no documented crop size. In such case usually no bleed and crop mark exists either.
How the problem could be resolved?
The modules shown below provide crop size, bleed and crop mark to the pdf.