The following information is derived from the plugin description.

A filter that uses the OCR plugin to prepare character mappings for fonts that would otherwise not support the mapping.

Some PDF files have a missing character mapping. Such mapping is required to translate from the visual text to machine readable text. The usual effect is a correctly displayed document with an apparently corrupt text in the comparison result. Furthermore the text is corrupted as well when copying & pasting from this document.

As a solution this filter will rebuild and correct the character mapping by using optical character recognition. The accuracy depends on the amount of text - with more text providing higher accuracy.