Portable Document Format (PDF) file created by Adobe Acrobat or another program that includes the Acrobat plug-in; supports text, images, forms, and other data; commonly used for publishing documents originally created in different formats. The ".pdp" extension is rarely used for PDF files, yet it is supported as an extension for programs such as Adobe Photoshop CS5. Since the ".pdp" extension is uncommon, you should use the .PDF extension instead.