This is a list of links to articles on software used to manage Portable Document Format (PDF) documents. 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.