Editing PDF directly on iPad/iPhone is a rather big job, because the standard API only supports showing it (and only a bit more.) If you want to do anything more, you need to invest a huge amount of time to implement generic pdf handling code.
There is an open-source library handling these, e.g. this one. I don't know if it fits your needs, though.
A better idea in my opinion is to create a native UI showing the data contained in the PDF file using the standard Cocoa-Touch UIKit, and create the PDF once the user is done with it so that the user can export it back. That way, you don't have to write a complicated PDF handling code.
In any case, it's not a good idea to show generic PDF on iPhone, because the screen size is so small (iPad is a different question, especially if you expect the user to be familar with the particular format of your pdf.). A dedicated UI would be much better.