Here's the problem with AirPrint: not nearly enough printers support it. Plus, there's that whole consumables thing. Aren't we supposed to be living in a paperless society?
Print to PDF for iOS solves both problems, allowing you to "print" e-mails, Web pages, and documents as PDFs right on your iDevice. It's a clever app, one that works much like PrimoPDF and other utilities for Windows.
In other words, it intercepts just about anything that uses the iOS print command and generates a PDF instead of a piece of paper.
All you do is run Print to PDF, then switch to any other app that supports printing. Tap the Print command, choose Print to PDF as your printer (a one-time task), and then tap Print again. The process may take a few seconds, depending on the length of the item. When it's done, you'll have the option of instantly switching to the app to view your new PDF.
Print to PDF cleverly organizes output into saved e-mails, saved Web pages, and "other" prints (i.e. stuff that comes from other apps, like, say, Documents To Go). If you need deeper organization, the app lets you add your own subfolders within these sections.
When viewing a PDF, you can switch to a text-only view, send it via e-mail, or open it in your preferred PDF viewer. (Hello, iBooks!)
The app automatically closes itself after a few minutes, so you do need to run it before pretty much every print job--a minor hassle.
In my tests, Web pages and e-mails converted to PDFs turned out beautifully. This is a great way to save items for later viewing or share them with others. Also, the app supports printing from one iDevice to another (via Wi-Fi), so you could print, say, an e-mail on your iPhone and view it on your iPad. I'm not sure how useful that is, but it's pretty cool.
Print to PDF costs $3.99 and works with most iDevices, not including the iPhone 3G and first/second-gen iPod Touch. I think business users in particular will find it a killer tool.