Hi
There are some good suggestions up there.
First although I think it may be less likely, check the File> Document Properties> Security tab, whether changes are allowed. You see this as a list item in the dialog box. If it says not allowed you are out of luck.
Then go to Tools> Advanced Editing and select TouchUp Text tool. Click any part of the text. If the cursor goes into edit mode, the file is editable. If not it is likely a scanned image that maximus suggests.
If it is the case, you can use the OCR function under the document menu and convert the document to editable text.
If it is not an image, you can use the said TouchUp text tool for text edits, and use the TouchUp Object tool for moving or deleting image objects.
hth
capt.