PDF (adobe.com)

Add or remove transparent colors

  1. Click the Preview, 2-Up, or 4-Up button at the upper left of the document window. In 2-Up or 4-Up view, click a view other than the original.

    Note: Though you can add or remove transparent colors in Original view, you won’t see the results until you display a preview.

  2. In the Optimize panel (Window > Optimize), select GIF or PNG 8 as the file format. Then select Index Transparency from the Transparency pop-up menu.

  3. At the bottom of the panel, click the Add Color To Transparency  or Remove Color From Transparency button .

  4. Click a swatch in the color table, or click a color in the document preview.