If you're working in Pixlr Editor and creating multiple layers to design a flyer or ad or header (or anything else), you may want to come back and update your file later. If so, you will want to save your image as a .pxd file. The PXD file format is the native file format for Pixlr Editor, and it preserves the layers of text, images, etc. You can save those files with a .pxd suffix to your desktop or to a Pixlr Library.
Want to know more about layers and how to preserve them? Check out the video Working with Layers from the Pixlr Editor Design Basics series.