To open a flattened version of a layered PSD file, go to File > Open and find the PSD file that you would like to open and click on it once to select it. Then hold down Shift Alt (Mac: Shift Option) and click “Open“. Photoshop will then ask you if you want to “Read the composite data instead.” Press “OK” and a flattened version of that file will open up.
This is an excellent way to open a large file that usually takes a long time to open. In case you only want to show it to someone or share it in an email or social media.
Always do a “Save As.” Saving the document will override the original file, and you will lose all your layers.