Adobe Acrobat 7 Professional - Manual (Page 172)

Saving Adobe PDF files
If you modify an Adobe PDF document in Adobe Acrobat Professional--for example, by
adding comments--you can save your changes by saving the PDF file or saving a copy of
the PDF file. You can also save changes to your work incrementally and then recover
those changes if a problem occurs.
Note: Saving a digitally-signed PDF document invalidates the signature.
Related Subtopics:
Saving document changes
Preventing and recovering lost changes