If you ever need to open a flattened (composite) version of a layered document, choose File > Open and navigate to the image. Then, hold Option + Shift (Mac) / Alt + Shift (Win) and click Open. A dialog appears with the message “Read the composite data instead?” Click OK to confirm.
You can use the same keyboard modifiers and double click a PSD file in the Operating System to open a flattened version of the file
Note: this shortcut only works if the file was saved with the Maximize (PSD and PSB) File Compatibility preference turned on.