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If the file is in your Gmail inbox, open the email containing the file and do the following: Tap the Add to Drive button for the attachment, usually in the middle of the preview icon. In the menu that pops up, select a folder in your Drive as the location for the PSD file. Select Add once you're done. This has a white icon with a yellow, blue, and green triangle in it that can be accessed from the Google folder. Once in your Drive, select the folder with your saved PSD file from the main menu. Simply tap on the preview icon for it. Since Google Drive supports a variety of formats, the file should open without any trouble.
Save the file you want to view to your Google Drive. Open the Google Drive app. Navigate to the file you want to view. Open the PSD file.