The distinction between the various functions is not entirely clear-cut; for example, some viewers allow adding of annotations, signatures, etc.
Some software allows redaction, removing content irreversibly for security.
Extracting embedded text is a common feature, but other applications perform optical character recognition (OCR) to convert imaged text to machine-readable form, sometimes by using an external OCR module.
[3] These are used by software developers to add and create PDF features.
These allow users to view (not edit or modify) any existing PDF file.