When you place any semi opaque object or an object that has a drop shadow applied to it in front of a 1 bit or grayscale image and print or export as PDF, the portion of the image that is under the drop shadow region that is flattened during output becomes corrupt.
There are three ways to address this issue:
- Open the grayscale image in Adobe Photoshop and save it as a copy.
- Choose Image > Mode > CMYK, and use only the Black channel.
- Choose Export > Layout as PDF and then click Options.
- In the Color pane, choose Grayscale from the Setup drop-down menu.
Assign an opacity setting of 99.9% to the imported picture.