Here’s a list of useful websites and ebooks for learning more about PDFs and PDFpen:
Smile’s PDFpen Support and FAQ page: You can find answers to common questions, and instructions on how to contact Smile for individual support questions.
Smile’s Help for PDFpen for iPad & iPhone: This PDF, also viewable from within the app, is good to have on hand. (Tip: save a copy that you can mark up with your own notes and additions.)
PDFtools.com’s PDF Primer: This white paper summarizes of the technological underpinnings of the PDF standard and the reasons that PDF was developed.
Prepressure.com: Customer support engineer Laurens Leurs maintains this site, which specializes in useful articles about prepress topics, including PDF.
The MacSparky Paperless Field Guide: This book contains lots of information about going paperless, and it provides even more ways to use PDFpen in that endeavor.
Take Control of Your Paperless Office: This book, by Joe Kissell, is the seminal work on paperless PDF workflows.