Merge or flatten layers

Merged layers acquire the properties of the layer into which they are merged (the target layer). Flattening PDF layers hides any content that is not visible when the flattening operation is executed and consolidates all layers.

Important: You cannot undo either a merging or a flattening operation.

Merge layers

1. Click the Layers button, and select Merge Layers from the Options menu.
2. In the left pane (Layers To Be Merged), select one or more layers to be merged. Ctrl/Command-click to select multiple layers, and click Add. Click Add All to merge all layers.
3. To remove a layer from the center panel, select the layer or layers. Ctrl/Command-click to select multiple layers. When your selection is complete, click Remove.
4. In the right pane (Layer To Be Merged Into), select the layer into which to merge the selected layers, and click OK.

Flatten layers

Click the Layers button, and select Flatten Layers from the Options menu.

Source：Adobe