Boxes where an opacity value or drop shadows have been applied undergo a flattening operation before a job is exported to PDF. When these flattened regions of a page are displayed in Acrobat or Preview, both applications render the flattened region using an anti-aliasing routine to smooth the transition between pixels. In order to view the exported PDF as it will print, you can turn off anti-aliasing in both Acrobat and Preview.

Follow these steps to remove the thin white line from the display in Acrobat and Preview:

In Adobe Acrobat:

  1. Go to Acrobat > Preferences (on Windows OS Edit > Preferences).
  2. In the Display preferences, turn off Smooth images and Smooth line art.

In Apple Preview:

  1. Go to Preview > Preferences. In the PDF pane, turn off Anti-alias text and line art.