You’re working on a brochure under a tight deadline, and upon opening the PDF you just exported from Adobe InDesign, you notice thin white lines around certain objects. Don’t miss your deadline spending too much time troubleshooting the InDesign file. More often than not, those white lines are simply a display glitch in the PDF caused by transparency flattening in the export process.
If you’re concerned, you can check your file in two ways. The first method is to simply open the PDF and zoom in and out – if the thin white line disappears when you zoom in and out, it’s just a display glitch.
The second method is to just run InDesign‘s Package command (Command + Option + Shift + P); when the report dialog appears, make sure you have no RGB images in use. When RGB images overlay CMYK images, transparency flattening problems can occur. If this is the case, then you definitely should convert those RGB images to CMYK before exporting your file as PDF.