From reading all of these questions, it appears to me you don't understand the concepts and I doubt that any of us here can do any better job at explaining them than the Help system that came with your software.
It might help to understand the difference between text, image files and software that is used to combine them into a document. Jpg and Tif are graphics files that you can put in a Word Document with your formatted text using the 'Insert' menu option. Adobe is a file distribution program that allows the creator many options for controlling how their file can be changed and distributed. The person that originated the file has the documents that made it so they can change it and put up the new version.
If you're trying to change your PDF, make a new one from the original documents. If you're trying to change somebody else's and it doesn't allow changes, the answer is, you can't.
Imagine if all documents on the Internet were in MSWord format. You could put something up there, I could download it and change what you said to something completely different. It would be chaos.
Answered Jul 31, 2010
Edited Jul 31, 2010