While the desire for accessible software is understandable, the technical reality of a 100MB Photoshop file is highly problematic.
You cannot compress a 4GB 16-bit, 300DPI billboard into 100MB without loss. Highly compressed means data destruction . The art is destroying the right data (invisible noise) and keeping the wrong data (edge detail and color accuracy). photoshop highly compressed 100mb
If you need to keep layers but want compression, save as a TIFF. In the save options, choose LZW or ZIP compression; LZW is lossless and very efficient for graphics with solid colors. While the desire for accessible software is understandable,