Layers > Merge Group
Merge Group
When you have finalized the content of a group, you can merge the layers to reduce the size of your image files. When you merge layers, the data on the top layers replaces any data it overlaps on the lower layers. The intersection of all transparent areas in the merged layers remains transparent.

Merging down causes all selected vector objects and bitmap objects to be flattened into the bitmap object that lies just beneath the bottommost selected object. The result is a single bitmap object. Vector objects and bitmap objects cannot be edited separately once merged, and editability for vector objects is lost.
1. Do one of the following:
 Choose Layers > Merge Group
 Click the Merge Group button on the toolbar
Related Information
Layers > Layer Basics > Groups
Layers > New > New Group
Layers > Merge Down
Layers > Merge Visible
Search the Internet