Editing, particularly adding pages to, a PDF document on the iPad is not a typical use of a PDF file (on any platform).
PDF is a format which tends to be for distributing documents in a common format which have been created elsewhere using all manner of software packages. Unlike a normal Office type document (word processor document, presentation, spreadsheet etc.), it's not normal to edit / add new content to the PDF itself once it is created. You would normally add / edit the original source material on the device where it was originally created, and then create a new PDF.
However, one thing PDFs and the apps which support them do lend themselves to well is annotation. So you can usually annotate an existing document by putting your notes etc. on top of the existing doc. The original source does not change, but your annotations are there for others to see. This is most commonly used to share your comments / thoughts on an original documents contents with others directly within the document itself, rather than creating separate notes.
The app I use on the iPad is PDF Expert, but there are literally dozens in the app store. There are none that I am aware of which would let you edit the PDF itself though. It's just not possible to know what software it was the created the PDF in all cases and therefore work out how the editor should let you edit it... The PDF could have been created by a complex Project Planning software tool, or be a printout of pictures from Photoshop. They needn't be just word processor style docs.
Does that make sense?
If you need more info, perhaps give us some more information about the PDF's you want to edit, their contents, where you get them from, and we'll see if there are any suggestions which would help...