🚀 go-pugleaf

Another thing to consider is that a PDF document can have usage rights applied (either by Acrobat or LiveCycle Reader Extensions) that allow Reader to do things it is not normally capable of. This is also known as Reader-enabling. Things like add comments, apply digital signatures, and saving documents that have been modified by filling-in form fields, commenting, and signing. This may have been the type of editing you had in mind.

To expand a bit on graffiti's response, a PDF can contain a wide range of content, including text, vector graphics, images, optional content, and various types of annotations including form fields, links, bookmarks, comments, multimedia (sounds, movies, SWFs, etc.), file attachments, etc, etc.

George