In Photoshop on the iPad, you cannot save a copy as PSD when your document exceeds the 2 GB limit for file size. This is a known limitation. Follow the workarounds suggested below.
Photoshop on the iPad
In Photoshop on the iPad, export your document in the TIFF file format if it exceeds the 2 GB file size limit.
For instructions, see Save and export cloud documents.
In Photoshop on your desktop, save a copy of the cloud document on your computer in Large Document Format.
For related helpful instructions, see Manage and work with Cloud documents in Photoshop.