A PDF document may contain the following standard types of signatures:
- Any number of approval signatures in signature form fields.
- At most one certification signature in a signature form field. It enables the author to specify what changes shall be permitted to be made to the document and what changes invalidate the author’s signature, cf. "Allowed and disallowed changes to a signed document" below.
- At most two usage rights signatures referenced from the PDF's permissions dictionary. Usage rights signatures shall be used to enable additional interactive features that may not be available by default in a reader. These signatures usually are not presented to the user but merely evaluated by the PDF reader program.