The easiest way to share files between devices is to use iCloud.
Turn on iCloud storage in the settings of PDFpen 2. Once turned on your files are automatically saved to iCloud.
On your Mac, if it is running OS X 10.10 (Yosemite), open Finder and you should see "PDFpen" listed under "iCloud". You can also open and save documents stored in iCloud directly from the PDFpen open/save dialog.
If you don't see the iCloud > PDFpen option in Finder you can track down your iCloud files in Finder by choosing View > Go to Folder (Cmd+Shft+G) and pasting in this location:
Another popular sync method is Dropbox. See our article Setup a Dropbox sync folder.