PDF Printer does what its name implies and more. The US$2.99 app allows you to open and read any document and turn it into a Portable Document Format (PDF} file. Even better, the app can convert email, photos, notes, and even web pages to PDFs.
Once you have captured the PDFs, you can save them, forward them via email or Dropbox, and open them in other apps capable of reading PDFs. You can also directly print the PDF if you have the hardware handy to do so.
PDF Printer also offers a scanner plug-in as a $0.99 in-app purchase, which is a limited time half-price sale. The plug-in lets your camera capture a document and quickly make turn into a PDF.
Using the app is straightforward, and help pops up when you first use it. I tried the scanner plug-in with positive results, and easily converted web pages and photos into PDFs. Mail can be converted by copying the mail to the clipboard and pasting it into the app. The PDF that resulted was sharp on screen and when it was printed to my laser printer.
The only thing missing from PDF Printer is the ability to scan text and make it editable. There are some OCR apps for iOS, but in my estimation they fall short in accuracy, so I understand why that capability is not included.
You can copy documents to PDF Printer via a USB cable using iTunes File Sharing. Once installed, PDF Printer appears in the "Open In..." list for many popular file types. You can download documents from your Dropbox account and upload converted PDF documents back or simply print them using AirPrint.
PDF Printer is not a universal app, and inexplicably, the iPad version costs $5.99. I would prefer the app be universal, or at least priced the same for either device.
There are some other options if you just want to scan documents. Genius Scan is a free and well thought of app that will scan using your iOS camera and convert to PDF format, but it doesn't support converting electronic documents.
PDF Printer works well, and is the best of the PDF apps I've looked at. It requires iOS 6 or later, and it's been optimized for the iPhone 5.