Editing PDF Documents
PDF is a format for output and review, not a source format intended for editing. Thus you can only do minor changes in it, such as:
* re-typing some text with the text-touchup tool
* move text or graphics — but only on a per-line basis!
* change font face, size, color for (part of) a line of text
* delete graphics with the object select tool
* insert a new line of text: Ctrl-click with the text-touchup tool
For all major editing, you should go back to the original source document and afterwards re-create the PDF output.