How to Merge PDF layers?

When you merge PDF layers, you consolidate one or more layers into another single layer. The layer properties of this target layer specified by the author of the original document prior to conversion to PDF are applied to the merged layers. Like flattening layers, merging layers cannot be undone. For this reason, you should always work with a copy of the original PDF saved under a different name, unless you’re absolutely confident about your merging and flattening desires. To merge one or more document layers into another, follow these steps:

1. Click the Layers tab on the Navigation pane to display the Layers palette.
2. Choose Merge Layers on the Options pop-up menu at the top of the Layers palette. The Merge Layers dialog box appears, as shown in Figure.
3. In the Layers To Be Merged list box on the left, select the layers you want to merge. To make multiple selections, Ctrl+click (Ô+click on Mac) each layer name, and then click the Add button to move the selected layers to the middle list box. You can remove layers from the middle list box by selecting a layer name and clicking the Remove button.
4. Click the Add All button to select all layers for merging and move them into the middle list box or click the Remove All button to delete all the layers selected for merging in the middle list box.
5. In the Target Layer to Merge Into list box, select the single layer you wish to merge the selected layers into.
6. Click OK to close the Merge Layers dialog box, and then click Yes to close the Warning dialog box and merge your selected PDF layers.

After you’ve merged PDF layers and display the Layers palette in the Navigation pane, you’ll notice that the target layer still appears in the Layers palette list, but the merged layers do not. To view the target layer by itself, click all the Show/Hide Layer buttons (the eye icon) attached to all layers but your target layer. When the target layer is the only layer showing, notice that it displays all the elements of the merged layers in addition to its own. To restore the layer view to its original state, choose Reset to Initial Visibility on the Layers palette Options menu.
When you’ve flattened or merged the PDF layers in a document, you do have one chance to undo the supposedly undoable. If your first response to flattening or merging layers in your PDF file is “oops!” choose File➪Close or press Ctrl+W (Ô+W on Mac) and when the alert dialog box asks you if you want to save changes to the PDF before closing, click the No button. The next time you open the PDF document, it appears in its last saved state.