Answered by Stephen Beals, Digital Pre Press Manager and Writer

Question:
What are “layered PDFs” and what are they used for?

Answer:
The newest versions of Adobe Acrobat support layering as found in Photoshop and InDesign. You can think of these as sort of three dimensional files. Although you can only see the "flat" file on screen, the layers are still there: essentially each layer can contain different data, text or images (even video and sound). Each layer can be turned on and off. You can view or print any combination of layers.

Since a layer can be just about anything, you have tremendous control over what is viewed on screen in the PDF document. You can do things such as applying different levels transparency depending on what part of the document you want to draw the viewer’s attention to and so on. As another example, you might have a map with arrows on one layer, roads on another layer, street names on another and so on. With a layered file, you could chose to "turn off" the street names, but leave the streets. Or you could have the names of the streets in English on one layer and Spanish on another. There are all sorts of applications for using layer functionality.

Older versions of Acrobat require that layered files be flattened when the PDF is created, but current versions can retain those layers when importing or viewing in the Adobe PDF format. The problem with flattening files is that once the layers have been flattened, you can never go back and edit them. When these files are output to a RIP for final rendering, they will, of course be flattened (converted to a single layer), but the original PDF document will retain all of the original layer elements.

One further use of this capability is the creation of "versioned" PDFs. In our map example, you may not want the file you send out to allow the viewer to switch from English to Spanish. From a single file, you could create multiple PDF files that would each contain different data. The file creator could turn off the English street names and turn on the Spanish names, then go ahead and flatten the file as a new document for a Spanish language version. Then switch language layers and save a flattened English version.

One caveat is that not all print engines can handle layers in PDF files and may have trouble rendering them. To assure proper output in print, look for a RIP that supports the PDF Print Engine. This engine understands PDF layers and transparency and will render them properly. The other option is to send a flattened file for print output.