Letter's filling in with gray color
I have come across this issue twice now. A client will send me an ad (pdf) created with inDesign. The copy has an outline turned on, the line size is set to 0, therefore invisable. When this happens you cannot see it on the pdf, but when it is sent to be printed the insides of the letters are filled in, but usually only on fonts that are less than 14 point. Not sure how I found this, a fluke for sure. Once the outline is removed the problem goes away. Just thought I would share.
