Do one of the following solutions:
Solution 1: Export as PDF 1.4 or later.
In the Export Adobe PDF General settings, select Acrobat 5 (PDF 1.4) or higher from the Compatibility options.
Solution 2: Turn off the Smooth Line Art option in Acrobat.
Choose Edit > Preferences (Windows), or Acrobat > Preferences (Mac OS) to open the preference dialog box.
In Categories > Page Display deselect the option Smooth Line Art, and then click OK.
Solution 3: Convert spot color to process color.
Choose Window > Swatches to open the Swatches palette.
Double-click the spot color to open the Swatch Options dialog box.
Choose Process from the Color Type pop-up menu.
Solution 4: Move the spot color to the top of the layer.
Change the stacking order of the objects, so that the object with the spot color is at the top, before flattening the image.
Select the object.
Choose Object > Arrange > Bring to Front.
Solution 5: Use layers to arrange the colors.
Place the spot colored object on a different layer, then arrange that layer above the layer that contains the transparent objects.
Solution 6: Set the opacity for the spot color to less than 100% and export to PDF again.
If your print shows dark lines on the printout of a created PDF file, then lower the opacity and create the PDF file again.
Select the object that contains the spot color.
Choose Window > Transparency to open the Transparency palette.
In the Opacity field, enter a value like 99.9%.
Export or print to PDF again