Transparency in PDF files

Transparency in PDF files refers to objects on a page (images or text) that are transparent.

Full transparency refers to the ability to make an object, or part of an object, fully invisible. Partial transparency is far more complex. Some proportion of both a foreground object and anything below the background needs to be mixed together.

The PDF standard supports transparency since PDF 1.4 and Acrobat 5. It is continuously enhanced by Adobe. When PDF documents with transparencies are printed on PostScript printers, the printing application must flatten the transparencies first, because PostScript does not support transparency.

Transparency can be used for many reasons:

  1. To show parts of objects that are normally hidden from view.
  2. To lighten images so that the text on top remains readable.
  3. To create a tint of a specific color or mix colors.
  4. To make underlying image objects shine through. Transparency enables features such as drop shadows, feathering, soft edges, blurs, and glows, as well as partial transparency of overlapping objects. Adding a drop shadow to text or images is one of the most popular uses of transparency.

How transparency gets added to a PDF?

The actions that add transparency to a layout:

  1. Feathering objects.
  2. Adding drop shadows to objects.
  3. Placing native files that contain transparency from Adobe Illustrator, InDesign, and Photoshop.
  4. Dragging & dropping (or copying & pasting) transparent objects from applications like Adobe Illustrator to Adobe InDesign.

Today, every mainstream file format and graphics application supports transparency.