Transparency in PDF files refers to objects on a page, such as images or text, which are transparent or ‘show through’. Transparency is typically used for shadow effects, to lighten (parts of) images so that the text on top remains readable, to make objects fade into another object or to create a tint of a certain color.
PDF 1.4 (the file format supported by Acrobat 5) was the first version of PDF that supported transparency.
It obviously isn’t a coincidence that this PDF release came to market around the same time that Adobe Illustrator 9 was launched. Transparency was ‘the’ big new feature in that release. Other applications like Corel Draw had already supported transparency for years but had to resolve to a trick to generate a printable file: when creating an EPS file or printing a drawing, the application had to remove or calculate the transparency effects. This process is called ‘flattening’.
Adobe also included transparency support in InDesign and they heavily promoted its use to the designer community. Suddenly prepress departments all over the world were flooded with designs that included transparency and their RIPs had to flatten this transparency to be able to make proofs or output plates.
How transparency gets added to a PDF
Sometimes designers are not even aware that their creation contains transparent objects. These are the actions that add transparency to a layout:
- Making an object transparent in a lay-out or design application (obviously).
- Adding drop shadows to objects.
- Feathering objects.
- Placing native files which contain transparency from Adobe Illustrator, InDesign or Photoshop.
- Dragging & dropping or copying & pasting transparent objects from applications like Adobe Illustrator to Adobe InDesign.
When do such transparent objects lead to a PDF file with transparency in it?
- PostScript does not support transparency. If a designer prints a file to PostScript and then uses Acrobat Distiller to convert this file to PDF, the resulting file will not contain any transparency.
- EPS files are PostScript files. They also cannot contain any transparency.
- PDF 1.3 files also cannot contain transparency. If a designer exports to PDF 1.3, the design application flattens the transparency.
- Only when the design application allows you to ‘Save’ or ‘Export’ to a PDF 1.4 or higher file format, can you end up with a PDF file that contains transparency.
How to Remove Transparency by Flattening the file in Adobe Application
Flattening an object means to merge all layers in the layer palette into one layer. This function is used to eliminate unnecessary layers when creating artwork in a design program. Having too many layers can be highly confusing and also can end up printing incorrectly when composing artwork.
To Flatten Layers:
In Adobe Illustrator:
1) Layer palette>Carrot on layers palette to view layer options>Flatten artwork
1) Layer palette>Carrot on layers palette to view layer options>Flatten image