Can I turn this around and ask you what exactly are you trying to do? Are you running into problems? If so, what are the symptoms?
In general, you can only edit documents that do allow modifications (e.g. the author did not protect the document). You can only edit text if you have the font that is used installed locally. You can only edit text that is stored as text in the document (vs. a scanned page, that does only contain a picture of text). And, you can only edit text in a "good" PDF that has all information that is required to allow you to edit the text.
Karl Heinz Kremer
PDF Acrobatics Without a Net
PDF Software Development, Training and More...
http://www.khkonsulting.com