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Windows users: When you Save As in Photoshop and choose the Photoshop PDF format, you have a choice of saving the file with either a PDF or a PDP extension. If you use the PDP extension, the file is a PDF but Windows knows to open it in Photoshop (as opposed to Acrobat or Reader). This can save time when using a page layout program's Edit Original (image) feature.
*Mac users should not be concerned about this since Photoshop PDFs always open in Photoshop, even if "normal" PDFs are set to open in Acrobat, Reader or Preview.