As a viewing program, Adobe Reader cannot make any changes to plain PDFs – it is not made for file creation or editing.
However, if you have Adobe Acrobat or Adobe LiveCycle Reader Extensions ES/ES2, you can activate functionality in a PDF that allows even Adobe Reader to handle that given PDF with advanced features, such as filling in forms, saving data into forms, commenting and annotating. These PDFs are known as “Reader-Extended PDFs”.
Sometimes when opening such PDFs, you will see the following warning:
This document contained certain rights to enable special features in Adobe Reader. The document has been changed since it was created and these rights are no longer valid. Please contact the author for the original version of this document
This means one thing: some program opened the PDF and made changes to the structure of the PDF itself – this is different from form-filling or annotating.
The change could have been made by an Adobe program (although Reader 8.1.0 displayed the message erroneously until 8.1.2 fixed it), another program (typically other PDF writers), or even automated programs that analyze and modify files.
At this point, only Adobe Acrobat can read the file, and re-apply the special features. If Reader has detected structural change however, it is advised to obtain a fresh copy of the form, as the changed version might not behave properly. Acrobat can export data from the damaged form, which can then be imported (by Acrobat) into the fresh form.
The fresh form can then be saved with this data in it, after which the special features can be applied to it.
The extended PDF can be returned then to the user to be used in Adobe Reader.