I know this thread is a bit old, but I wanted to share my experience and solution in case anyone else runs into this.
The other day, I spent many hours editing some photos for a client. I saved them all and closed Photoshop. When I tried to open the .PSDs, they all came up with this "Not a valid Photoshop document" error. I was freaking out. I could see the thunmbnails and previews with the MacOS Finder and could see the images just fine in Lightroom, but couldn't open them in Photoshop CC.
Finally, I remembered that these were NOT supposed to be .psd files! They were supposed to be TIFFs. What happened was, when I edited them from within Lightroom into Photoshop, they opened as TIFFs. However, when I did "save as", I wanted to name them similarly to some other files I had. I clicked on one of the existing files to copy the file name in the Save As dialog to make it easier to name the new file I was saving, but I didn't realize it was changing the file extention from ".tif" to ".psd" as well. The older MacOS (version 9 and earlier) didn't rely on file extensions fo identifying file types, so it didn't hit me right away that this would be the problem. When I changed the incorrect .psd file extension to .tiff, the files opened in Photoshop just fine.
SO... if you run into this "Not a valid Photoshop document" error, try figuring out if it's actually another file type (.jpg, .tif, .png, etc) by either getting the file info from another program or just experimenting with different file extensions. Chances are good that your file isn't actually corrupt, but just named improperly!
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