See the installation instructions here.
Install
To install PDFpen/Pro:
1. Download the standard disk image (“.dmg”) file on your Mac.
2. Double-click on the .dmg file to mount it. You will see the PDFpen/Pro application and the link to your Applications folder.
3. To install, drag the PDFpen/Pro application over to your Applications folder. This action copies PDFpen/Pro onto your Mac’s hard drive. 4. After installation, you may eject (unmount) the disk image and discard the .dmg file.
You will see PDFpen/Pro in Finder under Applications.
Important: If you have a Mac App Store version of PDFpen/Pro, you must install from the Mac App Store application.
NOTE: An internet connection is required for installation and registration.
Install PDFpen 9 and Earlier:
1. Locate the downloaded PDFpen or PDFpenPro application. It is likely in your Downloads folder.
2. Drag PDFpen, or PDFpenPro to your Applications folder to complete installation.