SECTION 3.5
121
Encryption
unlimited access includes the ability to change the document’s passwords and
access permissions.
•
Opening the document with the correct user password (or opening a docu-
ment that does not have a user password) allows additional operations to be
performed according to the user access permissions specified in the docu-
ment’s encryption dictionary.
Access permissions are specified in the form of flags corresponding to the various
operations, and the set of operations to which they correspond depends on the
security handler’s revision number (also stored in the encryption dictionary). If
the revision number is 2 or greater, the operations to which user access can be
controlled are as follows:
•
Modifying the document’s contents
•
Copying or otherwise extracting text and graphics from the document, includ-
ing extraction for accessibility purposes (that is, to make the contents of the
document accessible through assistive technologies such as screen readers or
Braille output devices; see Section 10.8, “Accessibility Support”)
•
Adding or modifying text annotations (see “Text Annotations” on page 621)
and interactive form fields (Section 8.6, “Interactive Forms”)
•
Printing the document
If the security handler’s revision number is 3 or greater, user access to the
following operations can be controlled more selectively:
•
Filling in forms (that is, filling in existing interactive form fields) and signing
the document (which amounts to filling in existing signature fields, a type of
interactive form field).
•
Assembling the document: inserting, rotating, or deleting pages and creating
navigation elements such as bookmarks or thumbnail images (see Section 8.2,
•
Printing to a representation from which a faithful digital copy of the PDF con-
tent could be generated. Disallowing such printing may result in degradation of
output quality (a feature implemented as “Print As Image” in Acrobat).
In addition, revisions 3 and greater enable the extraction of text and graphics (in
support of accessibility to users with disabilities or for other purposes) to be
controlled separately.