This article assumes that you are using a design program and your files are set up with bleed applied correctly. If this is not the case, see: How to correct bleed or What is bleed?

Sometimes the bleed area does not export correctly into the PDF file, this is a simple setting that has not been applied correctly.

In the PDF export options, go to the "Marks an Bleeds" setting and ensure that the bleed values are set to 0.125 inches on all sides.