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What are "Layers" in PDF?

Summary

In the PDF specification Layers are handled a differently than in most other software, such as graphics applications (Coreldraw, GIMP, PhotoShop, etc), and as a consequence have some limitations in how they are used. This article is intended to help explain what those differences are and how they affect the way "Layers" are handled our products.

What exactly is a "Layer"?

In Graphics applications (Not our PDF-XChange Editor, or any competing PDF editing software), Layers will often refer to a section of the content in a file that is distinctly on top of whatever layers are "behind" it, and behind the layers that are "in front" of it. In this scenario there exists a "Z axis" to determine the order of content, and any layer above the bottom layer would generally have a level of transparency so that the layers behind remain visible.  Whe you "bring forward" or "Send backward" content [image?] you are manipulating the "Z order" of the object(s).

In the PDF specifications, "Layer" is a term that was used by the Graphics industry and Adobe in particualr used it inthe context of PDF files.  It is actually a somewhat misleading description of what is happening, yet is commonly accepted as the "standard" name for this function. More appropriate terms for "layers" in the PDF format may be "Optional content groups", or "Visibility groups". There is no "Z axis" as there is no true order for layers in this format. Layers are instead intended to toggle the visibility of content, such as to quickly show or hide annotations, or to have a single document that displays for example a specific language, depending on the readers system locale.

How should I be using "Layers" in my documents then?

There are a few different use-case scenarios for Layers in a PDF document.  Note that when creating a layer, it is possible to configure some options that cannot be changed after creation, so it is always a good idea to look into these functions before beginning. For example, Layers can be set to automatically show and hide dependent on the selection chosen in a Radio Group form field that was created previously.

Here are some of the most common uses:

Commenting

"Layers" can be used to sort comments (highlights, pop-up notes, stamps, etc) into easily accessible "groups" these groups can be hidden and shown in the layers pane (Ctrl+L) as below.

To accomplish this, You can either select all the comments you wish to place on one layer, and then create the layer with the "Add selected comments to this layer" function:

Or you can select the comments afterwards, and change the "layer" option in the properties pane (Ctrl+`):

Note that each comment can only exist on one layer. To move a comment to a different layer, you must select the comment, as above, and select another layer from the properties pane.

Measurements

Because Measurement objects are comments they follow all the same rules.

With Measurements, you may find overlapping measurement objects or wish to show certain types of measurements in some situations. "Layers" allow you to group those measurements into groups that you can show and hide:

There are a number of additional uses here, for example, you could have sets of measurements using different units and quickly switch between them:

Languages

Layers can be used to display content in a specific language based on the user's location system settings.

While creating the layer, you can specify which language the layer should be automatically "shown" for:

This will automatically show the layer if the recipient's application language is set to the designated language (selecting partial language match will allow any language with the same prefix, such as en-US, en-Ca, and en-GB to all activate the layer). Usually the "program" language will match the system language, unless a user has intentionally changed this setting for their software.

Note that currently, this setting cannot be changed or reviewed after creating the layer, It is advised to include a hint to your settings in the layer name. If you need to change this setting later on, you will need to create a new layer.

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